RAW: Ambiguous
The rules are not clear on what, exactly, constitutes an “arcane spellcasting class” or a “divine spellcasting class,” probably because initially it was obvious. By default, assassins, bards, sorcerers, and wizards were the arcane spellcasting classes, blackguards, clerics, druids, and paladins were the divine spellcasting classes. There was no ambiguity: the former are the classes that cast arcane spells, and the latter are the classes that cast divine spells. Easy.
Then they printed things like Alternate Source Spell, Rainbow Servant, Sha’ir, and Southern Magician, which introduced ambiguity that hadn’t existed before. In these cases, you have spell slots from one class being used for either arcane or divine spells.
Does this new feature suddenly make the class into an “arcane spellcasting class” or “divine spellcasting class” where it wasn’t before? There hadn’t ever been a strict definition before, and they didn’t print one at this point, either. Some of these effects seem to try to include wording that prevents this kind of thing, but much of those rules are also unclear. For instance, consider this from Southern Magician: “The actual source of the spell's power doesn't change,” which Customer Service interpreted as preventing entry to mystic theurge. But it doesn’t really say that, does it? It says something about power source, which is unclear.
Unfortunately, there’s no direct, rules-as-written, “as it says on page xyz of Complete Shenanigans” kind of answer to this question.
Recommendation: Never
RAW is ambiguous, but what’s going to work well in-game is not: never, under any circumstances, should one be allowed to advance wizard spellcasting faster than the wizard does. That should never, ever happen in any game, and if you’re going to allow it you might as well allow Pun-pun.
Allowing these sorts of tricks to qualify for mystic theurge, and other prestige classes and feats that require one type of spellcasting or the other, is pretty clearly legal, RAW, and also usually far less troublesome. The only exception I’d be likely to make is the dweormerkeeper from Complete Divine’s web enhancement, but then I’d probably just ban that class outright.
Even allowing a divine-only prestige class to progress wizard spellcasting is almost-always not a problem. It’s the double-progression that should never, ever happen.
Duskblade 13/Crusader 1/Jade Phoenix Mage 6 seems to me to be much superior to Duskblade 20. You hit the really big duskblade class feature and retain your highest-level spells.
The issue is timing. The best levels to take Crusader are 5th, 9th, or 13th. Coincidentally, however, 5th, 9th, and 13th are also the levels where it is most painful to have a single level of crusader rather than being a single-classed duskblade, as those are the levels at which you get new levels of spells (and full-attack arcane channeling, in the case of 13th). You have two competing goals: get the next spell level (and full-attack arcane channeling) as soon as possible, while having maneuvers for as long as possible, since they are useful.
Therefore, I recommend taking the crusader level at 6th, 10th, or 14th; that is, I’d take the next spell level first, rather than the maneuvers. Here’s a breakdown of each option:
Duskblade 5/Crusader 1
You can take 2nd-level maneuvers with your first five maneuvers. This means, primarily, that you will have mountain hammer for the longest possible time, which is awesome. Other than that, though, you’ll probably still take a number of 1st-level maneuvers.
Duskblade 9/Crusader 1
You can take 3rd-level maneuvers and stances with your initial set; you can skip crusader’s strike for revitalizing strike, you can still take mountain hammer, and the amazing white raven tactics is available to you. Thicket of blades opens up as an interesting stance option; martial spirit is pretty small at these levels, and thicket of blades greatly improves your presence on the battlefield.
Duskblade 13/Crusader 1
Clearly if you haven’t taken crusader by this point, you should. Duskblade has little to offer, so you lose almost nothing at this point. The disadvantage of waiting this long is that you haven’t had maneuvers at all until this point. The advantage, on top of duskblade features ASAP, is divine surge, an excellent offensive maneuver.
The Awkward Bit
OK, so now that we’ve covered how to do it, there’s one other thing to consider: you cannot use arcane channeling and a martial strike at the same time. Arcane channeling requires a standard action attack or a full-attack, so the attack(s) you get from a strike don’t count. This makes strikes massively less useful to you than they would be to others. A boost or counter heavy selection of maneuvers solves this issue; you could do pretty well with defensive rebuke, shield block, and white raven tactics. But missing out on fantastic options like mountain hammer, tactical strike, the various healing strikes, and divine surge is a disappointment.
It also means you don’t have especially great mobility. You have invested 13 levels in duskblade to allow you to full-attack with spells, so you really do want to be making full-attacks. Crusader doesn’t help with that. Swordsage or warblade would help a little, since Tiger Claw is pretty good at that (sudden leap, arguably pouncing charge), but cleric for Travel Devotion or barbarian for Lion Spirit Totem (Pounce) might be better. Those options don’t lead into a great prestige class like jade phoenix mage, but they still might be better options.
The Other Thing
Finally, item-based healing is generally sufficient in 3.5. In-combat healing is rarely an optimal strategy, barring emergencies; the crusader is far better at this than other classes, but you shouldn’t usually need it. The aura you describe all-but-eliminates the worst emergencies, too. Wands and healing belts are quite sufficient for out-of-combat healing. As Jeor Mattan mentioned in comments, the real draw of actually having a cleric is the various options he or she would provide for removing status conditions, since items have HP-healing pretty much covered (and until you get heal, clerics’ HP-healing spells are pretty poor), and crusaders don’t cover that at all.
This is another situation where a level of cleric may be more useful to you: it would allow you to use wands of any cleric spells you like, with no Use Magic Device check. That will cover a lot your needs.
Best Answer
First, you are on the right track
I just want to confirm your thought processes here: spellcasters have many, many options, and while one of them can be the “fighting aspects of the game,” it is often better to leave those to other characters for two reasons:
Many other classes don’t have much to do outside from damage-dealing, the “fighting aspects of the game.” Doing so yourself steps on their toes.
There are other things you can do that other classes can’t, which means if you don’t, your party may lack them altogether.
Further, as you note, healing is not an all-day, every-day activity. In fact, the numbers work out such that you should generally prefer damage prevention over damage healing during combat. Damage that doesn’t happen doesn’t need to be healed, doesn’t risk someone dying before they can be healed, and mathematically, it’s simply more efficient (the same spell slot can prevent more damage than it can heal in most cases).
But buffing is done best with spells
Non-spell options for buffing are extremely limited.
Bard, and bard hybrids like skald, provide just about the only significant non-spell buffing options in the game. They are fairly good at this, but in order to get any significant buffing from these classes, means pretty much abandoning your oracle abilities. You can’t have both.
Dissident of dawn gives you some spells you wouldn’t otherwise have, and some (rope trick, mage’s private sanctum, mage’s magnificent mansion) actually help the party somewhat. But the only real reason to consider it is the communal mind blank you get access to as a 9th-level spell. If you get far enough to see 9th-level spells, that would be a great choice. Unfortunately, the entry requirements are brutal (two feats you will never use, plus being forced to take a spell you probably wouldn’t otherwise on a class with extremely limited spells known), so overall not recommended.
Envoy of balance is possible if you are neutral, and has some decent options for improving your channeling, but that’s healing, not buffing.
Arcane savant, bizarrely enough, works fine for divine spellcasters, and is... OK-ish. It loses a level of spellcasting, which is a huge blow, and while it gets some solid stuff, they’re not really worth it. None of them directly buff others, but the facility that the class offers with magic items potentially gives you access to a lot of buffs you wouldn’t otherwise have. The Dispelling Master feature is really solid.
Planes walker has some useful options, primarily in the abilities to counter summoning and teleport effects, and to use banishment. Loses a lot of spellcasting, though, not to mention your channeling.
Genie binder lets you create Lesser Elemental Seals, which can apply a minor all-day buff on someone. Only one person at a time can be buffed, though, and the feat requirements suck.
Holy vindicator has some rather-minor improvements to channeling, which isn’t buffing, and loses a ton of spellcasting levels.
Master of storms could benefit the party if you’re on a boat, maybe. Mostly, though, this is various blasting effects, i.e. stuff you don’t want or need, and again loses a ton of spellcasting.
Aaaand... that’s pretty much it. Those are the only prestige classes in the game that advance divine spellcasting and have anything remotely to do with buffing. As you can see, they mostly don’t have much to do with buffing.
Healing, on the other hand, is best done with items
Because healing is inefficient, many players only use healing out of combat and in emergencies. In both cases, it’s usually superior to use relatively low-power healing options: even 1 HP of healing will stabilize a dying ally, and out-of-combat, you can take the time to repeatedly use low-power, low-cost options.
Specifically, a wand of cure light wounds or (better, if you can handle alignment issues) wand of infernal healing are the most cost-efficient methods of healing in the game. Just owning one of these can allow you to heal the entire party back up to full HP after every combat, saving you a ton of spell slots that you could use for buffing.
Specific Recommendations
Multiclassing: Thou shalt not lose spellcasting
Spellcasting is by-far the best way to buff anyone, and there are almost-zero options for significant buffing outside of spellcasting (especially if you also want to keep your spellcasting). Thus, whatever class you take should advance your spellcasting.
But oracles have other class features that prestige classes, even those that advance spellcasting, aren’t going to advance, and usually have burdensome feat or spell-selection requirements. Really, straight oracle is your best bet.
Recommendation: Single-classed oracle.
Feats: Buff early and often
Improved Initiative is a rather-solid, if boring, feat for just about everybody. Buffing works a whole lot better if your buff is active before your allies try to attack or get attacked. For that matter, don’t be afraid to recommend to allies that they Delay until after your turn, if you’re both going before the enemy: that way they can act with the buff (obviously, this is not usually going to be worth letting an enemy go first, which is why you want to have high Initiative in the first place).
At lower levels, Extend Spell can be awesome, and it’s never terrible. Note, however, that extend metamagic rods are eventually pretty cheap.
At higher levels, Quicken Spell is worth... considering. The 4-spell-level increase is extremely painful, of course, which is why it’s often better to try to get quicken metamagic rods rather than actually take the feat.
If you cannot find or buy wands of cure light wounds, then Craft Wand suddenly becomes very desirable. Seriously, you want those wands. They will free up so many spell slots and allow your entire party to start every fight at full HP, guilt-free. Better yet, they literally just take one day to make, since they cost 750 gp and Craft Wand takes one day per 1,000 gp in the base price of the wand.
The arcane Eldritch Heritage is also very well worth considering. Improved Eldritch Heritage, in particular, is very nice, and Greater Eldritch Heritage allows you to pick up some spells from the sor/wiz list, which is excellent.
Recommendation: Buffing sooner (improving initiative), buffing longer or better (e.g. extending duration), buffing more often (e.g. quicken) are all possible. Eldritch Heritage can get you aspects of the arcane sorcerer bloodline, which enables better metamagic and gets you a few spells you’d otherwise miss. Craft Wand is very desirable if it’s the only way to get wands of cure light wounds.
The spells
Going through all of the oracle spells would be too much for an answer, but I will point you to this fairly-solid guide. I’ll also comment on some stand-out spells:
Infernal Healing – this is the most efficient healing spell in the game. The [Evil] descriptor is bizarre (3.5 had the same spell but unaligned, as lesser vigor), but if you can handle it, this is the spell to use for healing.
Cure Light Wounds – this is the healing to spell if infernal healing is out.
Heal – at high levels, the extreme potency of heal changes the conversation: unlike all other healing spells, heal can out-heal damage-dealers. Heal can be worthwhile to use in the middle of a combat, even if it is not quite an “emergency” situation.
Compassionate Ally – this takes an enemy out of combat, and gets an ally help, at the same time. [Mind-Affecting] is problematic, as is the Will save on something that works best against spellcasters, but if you can get it to land, it’s pretty awesome.
Life Pact – basically, no one in your party dies unless all of you die. Can be risky (a huge single-target effect could down your whole party, in theory), but ultimately should be quite a lot of durability.
(Communal) Protection from Alignment, Magic Circle against Alignment – these provide great protection against a lot of effects, namely those that establish ongoing mental control or possession. Note that this protection is not alignment-based.
Shield Other – mostly better than life pact, but this allows you to protect allies and ensure that healing is always available to those who need it most (because that’s most likely going to be you).
(Greater) Dispel Magic – every spellcaster ever should have both of these spells, period.
Magic Vestment – solid improvement to armor that you can hand out.
Greater Magic Weapon – excellent improvement to weapons that you can hand out.
Death Ward – eliminate most of a school of magic and most of an entire, common creature type. Must-have.
Freedom of Movement – shut down a smorgasbord of effects. Must-have.
True Seeing – expensive, so less must-have, but this is another effect that eliminates an entire school of magic.
Blasphemy/Dictum/Holy Word/Word of Chaos – out of theme, but these are extremely powerful effects. Worth considering.