I suspect there are no written rules about it.
A quick and dirty heuristic could be one of the following:
Skill ranks are training of the body and mind. You retain any skill point spent on mental skills (Int- Wis- and Cha- based ones). You use host's skill points on physical skills: even the Dex- based Thievery is stroed in a sort of body memory so that your fingers actually move by themselves when you are under pressure.
Skill ranks are applied knowledge. You retain all your skill points and ability modifiers on mental skills. You apply half (rounded down) of your skill points and the full host's ability modifiers on physical skills.
On the feat side, we could trace a rough line by stating that pure mental feats are those that don't enhance or modify any of the following stats:
- Hit points
- AC
- Damage reduction
- Damage dealt (if bludgeoning, slashing or piercing)
- Save DC of natural or extraordinary abilities driven through a melee or ranged attack (such as those of the Stunning Fist)
- Fortitude and Reflex saving throws
- Spell Resistance
- Forms of movement
- Physical skills
- Other statistics related to physical abilities (such as the maximum encumbrance)
Furthermore, any host's feat that provides additional maneuvers during melee or ranged attacks, or that provides or enhances other possibilities for changing position or location will be shut down. Possible examples:
- Step Up
- Rapid Shot
- Stunning Fist
- Power Attack
Many of the feats, as you already noticed, are very borderline. However, I'd prefer to be restrictive rather than permissive.
- Can a 5e simulacrum gain temporary hit points YES
- Can a 5e simulacrum regenerate an arcane ward YES
Simulacrum PHB p276
It appears to be the same as the original, but it has half the creature’s hit point maximum and is formed without any equipment. Otherwise, the illusion uses all the statistics of the creature it duplicates.
The simulacrum lacks the ability to learn or become more powerful, so it never increases its level or other abilities, nor can it regain expended spell slots.
If the simulacrum is damaged, you can repair it in an alchemical laboratory, using rare herbs and minerals worth 100 gp per hit point it regains. The simulacrum lasts until it drops to 0 hit points, at which point it reverts to snow and melts instantly.
The spell description about learning or becoming more powerful refers to implicit power i.e. level, training, hit points, spell slots. It does not refer to being the recipient of the effects from the use of spells or abilities that do not restore lost resources. Bless, Guidance, Enhance Ability, raging, Gauntlets of Ogre Power etc. all work on it. A healing potion does not. A Pearl of Power does not, and, to look at a different Wizard Tradition, Expert Divination (PHB p116) does not restore spell slots (RAW, though the actual description of the ability might lead to a house rule that the spell slot is not regained but actually represents it not having been used in the first place).
Temporary hit points PHB p198
Some spells and special abilities confer temporary hit points to a creature. Temporary hit points aren’t actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury.
Temporary hit points "aren’t actual hit points" so gaining them does not represent recovering hit points in any way, they are an effect of a spell or ability so the simulacrum can gain the benefit, just as they can wear Gauntlets of Ogre power to give them 19 strength.
Arcane Ward PHB p115
you can weave magic around yourself for protection. When you cast an abjuration spell of 1st level or higher, you can simultaneously use a strand of the spell’s magic to create a magical ward on yourself that lasts until you finish a long rest. The ward has hit points equal to twice your wizard level + your Intelligence modifier. Whenever you take damage, the ward takes the damage instead.
The protection gained from this ability is a ward that has it's strength measured in hit points. The simulacrum does not gain or regain hit points with this ability, it gains a ward, an ability's effect, where "Whenever you take damage, the ward takes the damage instead" with it's effect measured in hit points, and it is the the ward that regains strength when the simulacrum casts certain spells.
As an aside, the ward's strength is not temporary hit points so the Simulacrum, or any other creature with this ability, could have those at the same time too.
Best Answer
PFSRD:
Reading this straight, I believe as far as class levels are concerned, halving each classes' levels is the best way to do this, as long as the end result can come out legal. I feel the text clearly points this direction. If the creature has racial HD, those are halved as well. (If classes and racial HD, or just multiple classes, need to be rounded, some will need to round up to maintain the correct total HD/level after the operation. My best good-faith guess is that this should be caster's choice, unless for whatever reason another player should control the choices, such as if that player is going to play as the simulacrum or use it as a cohort.)
(Personal ruling note: If I were the one ruling, I'd probably make an exception only for prestige classes that clearly extend a specific class: if you can't qualify for that prestige class after losing half your levels in the base class, then the prestige class's levels would "count" as levels in that class to be lost. The examples below will reflect this non-RAW ruling, which I personally feel is as near the intent of the rules as possible. Other options might including ignoring/waiving prestige class requirements that can't be met and simply halving all classes' levels regardless; or always dropping prestige class levels first and only retaining the PrCs if more than half the original's HD are from prestige classes [maybe they're too complicated for the illusion to duplicate]. These options are no more or less rules-as-written [RAW] than my personal ruling, but may work better at some tables.)
Be careful while calculating levels for characters with prestige classes: Keep in mind that non-numeric requirements need to be met, too. If a Paladin X/Wizard Y/Arcane Archer Z is your target, of course you're going to check for the BAB requirement, but keep an eye on those feat requirements as well!
After you're done distributing levels, rebuy skills as near half the original totals as possible, except where prestige class requirements dictate otherwise. Then rebuy feats as desired, meeting PrC requirements first and choosing only feats the original has. Don't forget to remove attributes the original gained from levels/HD the simulacrum doesn't have.
Ridiculously long example text follows:
So, this is pretty straightforward for single classed characters with no racial HD: Halve their level, round down (per the general rounding rule). For characters with more than one class, or one or more classes with racial HD, matters get complicated. Here we go!
Two even-leveled classes, neither reliant on the other: Halve the target's levels in both classes. Sorc 6/Shaman 4 becomes Sorc 3/Shaman 2. Easy.
Even class, odd class, neither reliant: Halve the levels in the even class; halve (rounding down) levels in the odd class: Hunter 3/Ranger 6 becomes Hunter 1/Ranger 3. Ok.
Two odd classes, neither reliant: Halve the levels in both classes, rounding one of them up (in order to come out with the right total number of levels), caster's choice: Rogue 9/Wizard 5 can become either Rogue 5/Wizard 2 or Rogue 4/Wizard 3. Tougher.
Two classes, one reliant upon the other (prestige class): Check first to see if the character could be still legal if the operation for non-reliant classes is followed; if so, use that (Fighter 18/Stalwart Defender 2 becomes Fighter 9/Stalwart Defender 1). If not, count the prestige class and the base class together and get as close to the normal procedure as you can: Fighter 10/Stalwart Defender 10 becomes Fighter 7/Stalwart Defender 3 (because of the Stalwart Defender's BAB +7 requirement). A GM might need to get involved to resolve a single out-of-combat spell at this point.
More than two classes, none reliant on another: This is probably a specific niche build that the simulacrum won't be able to pull off. Consider choosing a new target. If you can't or won't, halve all even classes exactly and alternate rounding odd classes down and up, starting with down. If there's a level 1 class, and you can round it up (there's another odd class), do so to keep as much of the class make up of the original as possible. Bard 6/Cleric 6/Witch 1 becomes Bard 3/Cleric 3; Bard 6/Cleric 7/Witch 1 becomes Bard 3/Cleric 3/Witch 1; Bard 7/Cleric 7/Witch 5 becomes 4/3/2 OR 3/4/2 or 3/3/3, caster's choice. This should probably have a GM helping out, just in case.
More than two classes, one or more classes rely on exactly one class each: The ideal multi-and-prestige-classed scenario. If possible, just halve them all; if not, take from the prestige class(es) until the base class(es) meet the requirements. Barbarian 10/Stalwart Defender 6/Rogue 2 becomes Barbarian 7/Stalwart Defender 1/Rogue 1. At minimum, show the end result to your GM for approval.
More than two classes, at least one class relies on at least two other classes: The probable multi-and-prestige-classed scenario. As usual with prestige classes, check to see if you can just halve them all and still qualify for the prestige classes; if so, do so. If not, check to see if the simulacrum can qualify for the prestige class with the highest requirements (if more than one) with only half its total levels and still have at least one level to put into each prestige class; if so, distribute levels to classes until all requirements are met, then distribute levels to classes as close to half the original distribution as you can get. If not, drop the prestige class with the highest requirements and repeat the last step. Fighter 4/Wizard 5/Eldritch Knight 5/Arcane Archer 6 might actually become more streamlined by using Eldritch Knight levels to help meet Arcane Archer BAB requirements: Fighter 2/Wizard 5/Eldritch Knight 2/Arcane Archer 1. I can't promise there's no better way to do that one. If the requirements for Arcane Archer couldn't be met with a level to spare, you'd just ignore it completely: Fighter 4/Wizard 5/Eldritch Knight 5/Arcane Archer 2 would become Fighter 2/Wizard 5/Eldritch Knight 1. I am certain as I can be, having not met your GM, that he or she wants to be involved in this entire process.
Racial HD plus base class levels: Halve them all following examples already given, treating the Racial HD as just another class and ensuring the total end is half the total starting rounded down.
Racial HD plus prestige class levels: If the creature can still qualify for the prestige class with half the racial HD, do it. If not, take from the prestige class(es) to restore more racial HD until they qualify (as if the racial HD were a base class used to qualify for the prestige class).
Secondary question: As far as I can tell, other than reducing saving throw DCs and the like (from raw HD loss and/or from attribute loss), simulacrums' monster special abilities should be identical to the originals', except in cases where the original monster already has a natural HD progression or a template that changes based on HD. This might lead to some "obviously unbalanced" situations (such a damaging ability that doesn't normally scale to HD being far more powerful than the simulacrum should have access to, since the damage didn't scale down) that a GM may want or need to arbitrate, so even in a rather-strictly-RAW game you should check with your GM first if you're going to be using this spell on a creature with powerful built-in abilities not granted by number of HD or class levels. The comment question about genie simulacrum: nothing indicates that efreeti gain their wish-granting power at a specific HD total, so adding or removing HD should not affect it. Obviously, a GM probably shouldn't permit an efreet simulacrum to give a PC basically unlimited wishes; but RAW certainly seems to be written that way.