[Please no spoliers.]

I've played through about half the game with a combat-oriented character. But I've heard that it's possible to play through the game with a minimum of combat. My questions are:

1. What is the minimum number of unavoidable combat scenarios (a) to complete the main quest line, and (b) to complete all reasonable content?

2. How difficult is such a playthrough, compared to allowing yourself to fight?

Ideally, I'd like to play through the game a second time with a character with high Charisma/Intelligence/Wisdom and use the others as dump stats. Is this reasonable? Specifically, if I intentionally cripple my character for combat, even a single required combat encounter could make the whole idea no-go. For example, in Deus Ex Human Revolution, the few mandatory boss fights basically make pacifist characters infeasible, even though for the rest of the game you can avoid combat.

Thoughts?

Going completely and totally pacifist is viable aside from two unavoidable, albeit easy, fights that come to mind (can't recall the exact specifics you are asking about, sorry), but it will involve running away from a lot of combat; the stealth system that AD&D uses isn't all that good for facilitating a stealthy run, so you can't rely on that. You will be missing out on combat experience, but the good news is that the game showers you with quest experience, especially if you resolve potentially violent scenarios non-violently. Should you find yourself in a position where combat is necessary, you might find it advantageous to become a mage; the mental stats needed to make a powerful mage coincide perfectly with a high INT/WIS/CHA character. Besides, even if you aren't confident about your character's odds of survival in combat, the rest of the party shouldn't have that problem, if you decide to pick companions up.

I remember there was an area with two warring sides and one super-character waiting in the middle asking the player to help wipe them out both. And this had absolutely no diplomatic resolution, I just had two outright kill everyone (both of the clans, as well as that shady dude). That was for me a major putoff and I didn't play for much more after it. Which is a shame, considering how brilliant's the overall game!

That's actually (re)added by the Unfinished Business mod, assuming you mean the encounter in the Alley of Dangerous Angles. The clans and "super-character" are representatives of the Good, Evil and Neutral alignments respectively, and the idea is that you would defeat two of them (by killing the leaders of clans and/or the neutral character) allowing the remaining alignment to control the area. There is a peaceful "resolution" in refusing to kill anyone and just going on your way, though it obviously doesn't really solve anything.

I can't think of any other quests where there isn't a peaceful resolution, though, other than a couple of forced fights in the main questline. SPOILER Neither of these is to the death, though the enemy in one of them will make it appear to be. END SPOILER

Addressing the OP, the situation described above is one quest, readded by the Unfinished Business mod, which can only be resolved using combat. The combat in question isn't too hard; I got through it fairly easily with minimum physical stats, even before I'd become a mage. As far as I know, UB only adds one other quest requiring combat (at least in the components recommended here). While that combat is hard, it allows you to rest and regain spells each time you're defeated, while the enemies remain at the strengths you left them at, so you can whittle them down gradually.

Actually, that encounter is present in the unmodded game; I always wound up killing Rotten William's gang due them attacking me upon trying to weasel my way out of paying a toll, but I warned the other guys and spared them while killing the neutral party. Also, there is the quest to kill three of the Barking Dogs, but a pacifist character could just avoid doing it without any major repercussions.

I spared the Angels (the "good guys"), and in fact warned them of the nutter who had told me to kill them. After killing him, the only survivors were the Angels. The other gang I killed because they turned hostile due to me failing to bluff my way out of paying a toll. In exchange, the Angels granted me free passage through the Alley from that point on.

And the supercharacter?

"Good Guys" lived, "Bad Guys" and "supercharacter" deader than Elvis.