Even if the wording doesn't seem ambiguous to me, their meaning is.
As you correctly pointed out, you can perform a partial chargeD20SRD only when you are restricted into taking only a standard action or move action on your turn (for example, when you are staggeredD20SRD). Unfortunately, this is not the case of a readied action (with the exception stated below). So...
No, you can't perform (nor ready) a partial charge in normal conditions.
But, you can do it if odds are worse (that's the nonsense).
The only way you can ready a partial charge is when you are limited to a standard action or move action per turn. In that case you can use your standard action (the only action you have) to ready a partial charge (and you can perform it because of you are limited in the current round).
Partial charge seems to be a later addendum to the rules set, designed to provide a way for staggered creatures to attack while covering some distance.
I, ad a DM, will feel comfortable in letting anyone perform (and prepare) the standard-action partial charge at any time (this is actually similar to the 4th edition charge). But, to prevent a partial charge/then move away tactic, I'd state that your turn ends after a partial charge (as per 4th edition charge).
Hypothetical 1
The timing of a provocation
Casting provokes an attack of opportunity from someone who threatens you when you start casting. Note:
Concentration
Injury
The interrupting event strikes during spellcasting if it comes between when you start and when you complete a spell (for a spell with a casting time of 1 full round or more) or if it comes in response to your casting the spell (such as an attack of opportunity provoked by the spell or a contingent attack, such as a readied action).
There are two options here: either an attack between the start of the spell and its completion, or an attack of opportunity provoked (or readied attack triggered) by the action of casting the spell. That is, the full-round action you take on your turn to begin casting the spell.
So in the case of 1-round-or-longer-casting-time spells, people who move close enough to threaten do not get to take an attack of opportunity just for being there (though obviously they can attack using their turn’s usual attacks). They missed the provoking action that would have allowed them to do so.
Therefore, in this hypothetical, barring something like Spring Attack, he can’t attack while moving, and thus cannot attack the sorcerer. Attacks of opportunity never come into play.
Multiple attacks of opportunity with Combat Reflexes
Moreover, you cannot take multiple attacks of opportunity for a single provoking action, even with Combat Reflexes.
Combat Reflexes and Additional Attacks of Opportunity
[Combat Reflexes] does not let you make more than one attack for a given opportunity, but if the same opponent provokes two attacks of opportunity from you, you could make two separate attacks of opportunity (since each one represents a different opportunity). Moving out of more than one square threatened by the same opponent in the same round doesn’t count as more than one opportunity for that opponent.
Any given action can provoke at most once. This is most relevant when moving about in someone’s threatened area: they get one attack of opportunity, not one for every threatened square you leave for that same action.
Hypothetical 2
No Spring Attack, no threatened area, no attack of opportunity
Without Spring Attack or similar, this is clear:
Threatened Squares
You threaten all squares into which you can make a melee attack,
Without Spring Attack, the monk cannot attack while moving, and therefore threatens nothing. Since he cannot threaten, he cannot take any attacks of opportunity.
Spring Attack and a threatened area: still need a provocation
Now, the case of Spring Attack (or similar). The monk does threaten, and could take an attack of opportunity if one were provoked. Is one?
Readying an Action
If the triggered action is part of another character’s activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action.
The charger does not act during the monk’s readied action. As a result, he does not move, and since he does not move, he does not leave any square the monk threatens. Thus, there is no provoking action that the monk can take advantage of.
Provoking the charger
As to whether the charger could have taken an attack of opportunity had the monk not used Tumble to get past him, yes, he could have, because he gets to layer yet another exception here:
Making an Attack of Opportunity
An attack of opportunity “interrupts” the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).
So even though the charger does not generally get to act while the monk resolves his readied action (the charger’s charge is put on pause until the monk’s readied action resolves), the charger gets to interrupt the monk’s interrupt, putting his move on pause, and take the attack of opportunity. After the attack of opportunity is resolved, the turn continues, that is, the charger goes back to pause and the monk resolves his move action, and then the charger’s turn continues.
Bonus on attacks of opportunity during a charge
Finally, one last thing to note:
Attacking on a charge
After moving, you may make a single melee attack. You get a +2 bonus on the attack roll and take a −2 penalty to your AC until the start of your next turn.
I would argue that the charger’s attack of opportunity, assuming the monk had not used Tumble, would benefit from this +2. Since the charger has moved, the “after moving” clause is satisfied.
Title Question
In D&D 3.5, can a character take Attacks of Opportunity while moving without stopping?
In the hypotheticals proposed, no. In general, though? To say “no” conclusively, we would need a rule that explicitly says as much; no such rule exists. To say “yes” conclusively, we would have to demonstrate a situation in which an attack of opportunity happens for a character currently in motion.
Such a situation:
A spellcaster readies an action to cast a spell when someone enters a square within 10 ft. of himself.
An enemy of the spellcaster has the Spring Attack feat, and also has a reach that includes squares 10 ft. from her own position (e.g. Small or Medium with a reach weapon, Large without a reach weapon, etc.). She moves, one way or the other, and her movement takes her into a square 10 ft. from the spellcaster.
The spellcaster’s readied action is triggered, and he takes it. The spring-attacker’s movement is paused so the spellcaster can resolve the readied action.
The spellcaster decides, for whatever reason, not to cast defensively (maybe he does not realize that the spring-attacker has 10-ft. reach, or maybe she has the Mage Slayer feat). Thus, when he casts his spell with his readied action, he is performing an action that provokes.
The spring-attacker, whose movement is on pause while she is in a square 10 ft. from the spellcaster, has a threatened area that includes the spellcaster. She is therefore entitled to take an attack of opportunity, and does so.
The attack of opportunity is resolved; an attack roll is made, and if it is high enough, damage is rolled. If damage is dealt, the spellcaster must make a Concentration check to keep the spell. Regardless, at this point the attack of opportunity has been resolved.
After the attack is complete, the readied action continues. This involves either casting the spell, or simply ending the “pause” if the spellcasting has been disrupted. Either way, the readied action has been resolved.
The spring-attacker’s original movement is unpaused. She now gets to complete her movement.
Best Answer
Questions & Answers
Question: A creature wants to take a standard action to ready, setting the action and condition as I take a 5-ft. step toward the foe and make a standard melee attack if a foe with reach weapon gets within 10 ft. of me. Is this a valid ready action condition and action?
Answer: Yes, if the DM says they are.
Assuming the DM agrees, because "the triggered action is part of another character's activities, [the creature who took the ready action] interrupt[s] the other character" (PH 116). Thus the creature who takes the ready action needn't worry about making the ready action's standard melee attack before a creature's met the condition somehow. The creature who met the condition is engaged in an activity--moving. Were the condition set to an activity that defies interruption, the ready action's action would occur before that activity. In either case, however, the creature who takes the ready action's action has its place in the initiative order adjusted to before the creature who met the ready action's condition.
Question: From the previous question, can the creature that met the ready action's condition still make an attack after the ready action's action is resolved?
Answer: Yes, depending on his available options and weapons. The creature who met the condition can adjust his behavior based on the resolution of the ready action's action.
For example, the creature who met the ready action's condition by moving 5 ft. can, after the ready action's action's resolved, continue taking a move action to move up to his speed or state that 5 ft. of movement is his 5-ft. step and take a full action to make a full attack.
For example, the creature who met the ready action's condition by taking the charge action can take a free action to drop his reach weapon and attack (gaining the bonuses for making a charge) with a weapon both that he can ready as a free action and that can hit an adjacent foe (e.g. an unarmed strike).
Question: I'm considering the following house rule: When a creature's ready action's conditions are met, the creature that takes its ready action's action and the creature that took the action that met the ready action's conditions both take their actions simultaneously. What are this house rule's repercussions?
Answer: First, this house rule puts the burden of deciding precisely when actions occur onto the DM. As that's one of the most important parts of any game, it's a burden--for good reason--the game normally assumes instead.
Second--and probably the rule's intent--, the house rule forces creatures to take actions they wouldn't otherwise take.
In Example A, Countess Cecelia saying she's making a ranged attack doesn't commit her to making a ranged attack. Countess Cecelia's taken no actions when Lady Alexa's conditions are met, and because Countess Cecelia "is still capable of doing so, [she] continues [her] actions once [Lady Alexa] complete[s her] readied action" (PH 160). Example A means Lady Alexa must play smarter (e.g. not zipping up to crossbowmen who have obvious and readily available melee weapons).
In Example B, Countess Cecelia is required to take her action because actions occur simultaneously. Countess Cecelia, then, must make the ranged attack at some DM-determined point despite the conditions surrounding that ranged attack changing. Example B means Countess Cecelia, through no fault of her own, may--depending on the DM's judgment--behave foolishly (e.g. firing her crossbow at an adjacent creature who's waving a sword in her face when Countess Cecelia has a perfectly good longsword at her side).
Question: I'm considering the following house rule: Creatures can't take a 5-ft. as part of the ready action. What are this house rule's repercussions?
Answer: I don't know. I've never played in a campaign with such a house rule, and the ready action hasn't been used to such a degree that revisiting it was necessary in any campaign I've played in or DMed.
That said, I couldn't even find discussion about this specific house rule, but, tangentially, if considering such a house rule, there's one group that removed everyone's ability to take a 5-ft. step when threatened, and that group was pleased by the results. Further, one discussion proposes eliminating the 5-ft. step altogether. Those could be of interest.
Question: A creature takes a standard action to ready, setting the action and condition as I take a 5-ft. step away from the foe and make a standard ranged attack if a foe moves adjacent to me. Is this cheesy?
Answer: No. In fact, it's a valid stalling tactic, especially against foes with limited options.
First, the creature that takes a ready action consistently won't, under most circumstances, ever be able to take a full action, and that includes making a full attack. This by itself makes the ready action of limited to use to creatures reliant on full attacks to inflict lots of damage to their foes, as many archers are.
Second, after the creature that took the ready action's action resolves its ranged attack, if the creature that met the ready action's condition can continue his movement, he may. That is, unless that creature's already taken what he deliberately called as a 5-ft. step (e.g. to avoid attacks of opportunity from a second foe), he can continue moving up to his speed and, if actions remain, take a standard action to make a melee attack against the archer.
Third, the archer--unless, in addition to wielding the bow, he's also simultaneously wielding a weapon that permits him to threaten an area--doesn't threaten an area and can't make attacks of opportunity. After the archer uses such a tactic a time or two, the archer risks his foe changing tactics from standard attacks to, for example, grapple attempts, sunder attempts, or trip attempts.
The ready action depends a lot on the DM
Some DMs are comfortable with the ready action being extremely broad, while others are uncomfortable until every variable is defined. Most are in between. Readying an attack against a creature or event is usually sufficient, but the DM can demand greater specificity, mandating creatures who ready determine their actions with regards to weapons, ammunition, and use of feats and other special abilities. Further, the DM can demand the condition include occupying a location on the battlefield or a specific foe when multiple foes are available.
I'd suggest, rather than using house rules to modify the ready action that the DM instead carefully examine how PCs are abusing ready actions then mandate more specific actions and conditions.
Many DMs would probably find this condition perfectly reasonable, but if players are abusing the ready action to such a degree that house rules are being considered, the DM can mandate the ready action's action and condition be more refined.
As an example, the DM can say that to pick a creature's action as a condition, the creature who take the ready action must pick as a condition a specific action (e.g. charge, grapple, standard attack, full attack).
This has precedent. Several weapons say to use the ready action to brace against a charge. For example, "If you use a ready action to set a halberd against a charge, you deal double damage on a successful hit against a charging character" (PH 118).
Thus a creature can take the ready action, setting the action and condition I make a standard attack with my halberd if a creature charges to within my reach. Such a condition is unmet, for example, by creatures who take move actions to move up to their speeds within the creature's reach or who take full actions to run within the creature's reach.
Thus under a DM who mandates it, Lady Alexa's ready action would be I take a 5 ft. step toward Lord Bob and make a melee attack if Lord Bob charges to within 10 ft. of me. Lord Bob, then, could take a double move or even take the run action to approach Lady Alexa without meeting the conditions.
(I don't think this is necessary in most cases. It would take a lot of abuse of extremely broad conditions to make me implement it. The most common use of ready actions in most games I play and DM is distracting spellcasters, which has specific rules regarding the ready action. Most of the time when my players want to ready what they really want to do is delay.)
It's also up to the DM (or, with a generous DM, maybe Lady Alexa's player) to describe to Lord Bob how Lady Alexa's actions appear on the battlefield:
It's entirely the DM's call if Lord Bob knows what Lady Alexa's done, and if Lord Bob knows what action Lady Alexa's taken Lord Bob won't charge. He could flee, taking the run action. He could take a double move toward Lady Alexa and see what happens. He could wait. He could draw a hidden weapon. He could cast a spell.
But let's say the DM goes with number 1, above, and Lord Bob totally does what Lady Alexa knows Lord Bob always does: charge in with his glaive.
Lord Bob could've made an attack with an unarmed strike--switching grips is a free action and it's Lord Bob's turn--and provoked an attack of opportunity from Lady Alexa, but gaining the charge's bonus on his unarmed strike. Also, had Lord Bob the feat Quick Draw (PH 98) and a weapon capable of threatening adjacent squares he could've made an attack with that weapon, also receiving the bonus for his charge, dropping his glaive as a free action. He could've had armor spikes. A spiked gauntlet, even. But he doesn't.
"Lord Bob shouldn't have to wear spiked armor!"
You're right, he shouldn't, but Lord Bob should be aware that his chosen fighting style leaves him vulnerable to careful, patient foes, and that he's a reach fighter without a way to attack adjacent foes. That's a weakness his foes can--and will--exploit.