Since these games are all based on AD&D (either edition), I figured maybe it would be helpful to have a thread where people can ask rules related questions about AD&D and get them answered by those of us with in depth experience of the games.

Might make a nice, handy resource.

Here's a few questions about things the games handle inconsistently:

1. Can Fighter/Clerics use swords? (In the SSI games (including 2e games) they can, in the IE games they can't.)

2. What spells do Mages automatically learn (without having to learn them from spellbooks/scrolls)? In particular, how many spells are learned at level up? (Again, not all games implement this the game way; the Gold Box and Dark Sun games give you 1 per level; EoTB and Dungeon Hack give you none; and the IE games give you none.)

3. Exactly how is Vampiric Touch *supposed* to work? Can it provide permanent healing (as opposed to just temporary HP)? Does it stack with itself?

Okay, so the "official" answers (as you say, often implemented in different ways)

1: Fighter/Cleric (meaning multi-class demi-humans)

2nd edition is clear that multi-class priests MUST obey the priest restrictions, meaning no swords.

In 1st edition, half-elves are listed as NOT obeying the restrictions while a half-orc fighter/cleric must.

FOr dual-classing, in AD&D, weapon restrictions apply only to functions that are class specific, so a cleric turned fighter could use a sword just fine (as no cleric function is dependent on a weapon, its more for things like thief backstabbing).

2: In 2nd edition, you do get a new spell upon levelling up. I don't have a 1st edition DMG at hand right this moment.

3: The hit points gained are temporary, expiring after about an hour, though it can take you above your normal maximum.
Nothing seems to indicate it can't be used repeatedly.

How does the Cloack if Displacement really work?

Do the Saving Throws bonuses from magical Rings and Cloaks work with magical armour?

Does the Paladin really have Protection from Evil 10' that affects adjacant allies?

I can only check magic items for 2nd edition ATM.

Cloak of displacement makes the first melee or missile attack miss automatically. All subsequent attacks are at +2 to Armour class and saving throws.

Ring of protection WILL grant its saving throw bonus while wearing magical armour but it will NOT add its armour bonus if you are wearing magical armour.
(I have a feeling this was different in 1st edition, I'll clarify that as soon as possible).

Paladin protective aura is indeed 10 feet. In 2nd edition, it specifically grants a -1 to hit penalty of "evil" and "summoned" creatures, while in 1st edition, its the exact same effect as the Protection From Evil spell.

(Baldurs Gate managed to somehow come up with a completely different approach to handling this)

Oh crap, sorry! The problem with sussing out the paladin aura is that it'd be nearly impossible to tell, because the game doesn't display dice rolls and I don't remember off the top of my head if the enemies ever employ summon spells in the series.
The manual for Curse says paladins do protect allies, but a -1 to hit is so minor, you might never notice the difference even if it does apply to adjacent allies.

Ditto for saving throws, since we don't have dice rolls displayed.

Here are some likely trickier questions about AD&D rules.

1. Do Power Word spells check current or maximum HP? (For example, if a character has 61 max HP, takes 2 damage (leaving her with 59 HP), and is then attacked by Power Word, Kill, does she die?)

2. Do temporary HP (from Aid or Vampiric Touch) affect Power Word spells? (Suppose a character with 59 HP gains 3 more HP from Aid and is then hit with Power Word, Kill. Does she die?)

3. (This question is specific to 1e) Suppose a male character has more Strength than would be allowed for a female character of his race. The character then dons a Girdle of Femininty/Masculinity. Does the character's Strength change? (Note that this becomes a non-issue in 2e because the sexist female Strength rule was removed in that edition.)

4. Can a charmed character be made to attack her allies, or is that 1st level spell not that powerful?

5. Suppose a level 6 Cleric with 18 Strength casts Draw Upon Holy Might. Is the character's Strength 20 (as in Baldur's Gate), 18/51 (as in Icewind Dale), or some other value entirely? (Also, in what book was that spell printed?)

6. How does Enlarge affect Strength in the tabletop game?

2nd edition makes it explicit that CURRENT hit points are used.
1st edition does not say explicitly either way. Based on the above answer, I'd say that they do affect it.
The description of f.x. vampiric ttouch doesn't indicate that the temporary hit points are unique or different in any way other than being temporary. That is actually a fantastic question.

I looked through every incident I could find in 1st edition of a character being changed in anything else (polymorph, reincarnation and a few others) and there's no set rule for this anywhere, meaning it'd be up to the DM.

I'll try to look through old issues of Dragon Magazine and see if they ever discussed this. This is surprisingly unclear.
The description (in both editions) says that the chracter will regard the caster as a "trusted friend and ally" but that does not remove any existing loyalties.
However, it also suggests that anything said by the caster will be viewed in the most favourable way possible.

So that would come down to roleplaying.
((Purely personally, I would say that creatures with little loyalty to each other like orcs would easily fight their former comrades)) It's from Tome of Magic.
From an online transcript of the spell, the example specifically gives a strength of 21 as an example. It does not affect Strength directly. Instead it increases melee damage by the same proportion, so if you were enlarged 40%, all attacks do 40% more damage.
1st edition doesn't give an exact effect, leaving it to the GM.

Note that a level 6 Cleric only gets +2 to an attribute, as it's +1 per 3 levels (up to a maximum of +6). Also, exceptional Strength (ie. the scores 18(01) to 18(00)) is generally considered a Warrior only feature and does not interact with spells except when explicitly stated otherwise.

*All my answers are based on AD&D (1e) PHB and DMG* (I’m not nearly as familiar with 2e) 1. Yes in all cases. (PHB p32)

2. At start: Read Magic, +1 Offensive, +1 Defensive, +1 Miscellaneous (the book has specific lists upon which the DM is to roll randomly). At level up: one spell (not specified if it's player's choice or random). (DMG p39)

3. I cannot comment because this spell doesn't exist in 1e AFAIK. 1.Current hp. (Explicitly stated only for “Power Word, Stun”, presumably the other two spells work the same way). (PHB p88)

2. Not explicitly stated. However, as in the answer directly above, the Power Word spells seem to use current hp, not maximum hp.

3. Not explicitly stated.

4. Not explicitly stated. The exact wording is: “The spell does not enable the [caster] to control the charmed creature as if it were an automaton, but any word or action of the [caster] will be viewed in its most favourable way. Thus, a charmed creature would not obey a suicide command, but might believe the [caster] if assured that the only chance to save the [caster’s] life is if the creature hold back an onrushing red dragon for ‘just a round or two’… If the [caster] harms, or attempts to harm, the charmed creature by some action … the charm will be broken automatically.” So it’s really a DM call on a case-by-case basis. (PHB p55,56)

5. I cannot comment because this spell doesn't exist in 1e.

6. "Enlargement causes increase in both size and weight" ... It will “increase the size of a living creature … by 20% per level of the experience of the magic-user, with a maximum additional growth of 200%.” “The maximum volume of living material which can be initially affected is 10 cubic feet … per level of the magic-user” “.… weight, mass, and strength are [increased]” “A person 12’ tall would be as an ogre, while an 18’ tall person would actually be a giant for the duration of the spell.” (PHB p65).

The last one is very interesting. Does the spell increase each dimension of the target, or the volume? If it increases each dimension, then (assuming a target who is 6' tall) a 5th-level caster could make them ogre-sized (12') with ogre strength (18/00 based on Gauntlets of Ogre Power), and a 10th-level caster could make them giant-sized (18') with giant-strength (19-24). This is the proper interpretation in my opinion, as it matches with the examples given at the end of the quote above. However (just for fun) if volume is what is increased and not height, then a 10th level caster would increase the target’s volume to 3x what it was originally but this would only result in a height increase of 44% (mathematically). So if someone started out as 6’ tall and 160lbs and had this spell cast on them by a 10th level caster they would end up as roughly 8’8” and 480lbs, not even ogre sized!