(1). Race (Elf, Dwarf, Halfling) as a class? Yes or no?
Yes, with the caveat I'm using ACKS to do so, so there's more variety. Is that substantially different than split race/class? Probably not, and if you use my answer to #4 below, having an Elf Fighter is just a matter of XP table recalculating. Bolting the race on afterwards or building it from the start isn't a lot of difference in the end.
(2). Do demi-humans have souls?
I'm currently working with just Elves and Dwarves, and the difference betwixt the two is less than usual. Also, not regular souls. They're from elsewhere.
(3). Ascending or descending armor class?
Ascending, because I have that thing where I'm bad at subtraction on the fly. Is that a condition? Is it just called 'dumbness'?
Actually, it's not so bad anymore - d20 really needed ascending AC because of all the fiddly situational bonuses.
(4). Demi-human level limits?
While I doubt it will come up, I don't really see the point. See here:
Elven Spellsword, 11th: 770,000 XP - fighter/mage (actually capped at 10th)
Human Fighter, 11th: 490,000 XP
Human Mage, 11th: 610,000 XP
Human Cleric, 11th: 400,000 XP
Human Thief, 11th: 380,000 XP
Figure PCs getting to 11th level is a long campaign. Your demihumans are already virtually capped just by virtue of leveling slower - letting the Elves and Dwarves continue to 14th just means that your players with human characters have retired to run their domains and they're now playing their henchmen of whatever level best matches the Elves and Dwarves, if the game just doesn't end there with everyone retiring.
To take an extreme example. the Elven Spellsword would require 1,280,000 XP to reach 14th level. That's another 430,000 XP. If he's in a party with a Mage, Cleric, Fighter, and Thief, each at 770,000 XP, let's see what happens:
First, the Cleric and Thief already retired - the Cleric 70K XP ago, and the thief 90K XP ago. They've been replaced with 11th level versions of themselves. Let's assume each replacement is at the minimum for the level they appear at, and everybody retires immediately when they hit 14th level, and everybody is gaining the same XP amounts.
Here's a table. There's probably errors, but the general gist of the thing stands:
The Elf retired 9 other characters getting from 10th to 14th - 10 if you count that the 4th Thief gets to retire the same day as the Elf! (ACKS XP Math is weirdly cyclical, or I'm bad at stuff) He ends his career several steps away from whom he started it with.
How many times is that really going to go on in a group? If they wanted to, and everyone was having fun, why not?
(5). Should thief be a class?
There is room in the source fiction for a class, that while she does not fight as well as a dedicated warrior, has some mechanical advantage in guile or gumption that makes up for it. Thief has been the most common of those.
(6). Do characters get non-weapon skills?
Yes, again, room in the fiction.
I'm not making any comment on how #5 or #6 goes about their game mechanics, cause there's a brazillion ways to do it.
(7). Are magic-users more powerful than fighters (and, if yes, what level do they take the lead)?
How, exactly? Man-to-man combat? I'd say that's maybe true in some range between 5th and 8th, given the mage has had ample time to prepare just the right spell load-out.
Domain-level play? I'd start to side with the Fighter - once he starts bringing massed troops, the advantage ought to be his, unless the Mage has spent the same resources doing the same, and not magical research.
(8). Do you use alignment languages?
No. There's a type of campaign world that probably works in. The one I'd run would be where it's a world heavily influenced and populated by the denizens of the Upper and Lower Planes, and it just so happens that only Lawful people can understand what the Lawful outsiders say, and vice-versa.
That being said, I'm currently bending in the winds of a more Swords & Sorcery & Planets inspired milieu, and I may come around to the idea.
(9). XP for gold, or XP for objectives (thieves disarming traps, etc...)?
XP for gold & awesome.
(10). Which is the best edition; ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E ADD, 2E ADD, 3E ADD, 4E ADD, Next ?
Whichever one you can play with the people you want to play with. My brainspace is in ACKS, and a hacked up version of it is what I play with the kids, and that's B/X on the paternal side. I play in a game with a few friends that's d20/3E modified by Trailblazer and the E6 rules (and not really played very tightly tied to the rules even then)