(1). Race (Elf, Dwarf, Halfling) as a class? Yes or no?
Definitely yes. Demi-humans in the Barbaric Frontier are a very small group, so the issue of one culture one race is non-existent.
(2). Do demi-humans have souls?
Yes like all the sentient races of the Barbaric Frontier.
(3). Ascending or descending armor class?
Definitely ascending AC, I hate combat tables,
(4). Demi-human level limits?
Yes, in the Barbaric Frontier all PCs (demi- and humans) can achieve 10th level and then retire.
(5). Should thief be a class?
Definitely no, all classes can steal, pick locks etc..
(6). Do characters get non-weapon skills?
Yes, in a way, I'm using ST-as-Skills as in Crypts & Things.
(7). Are magic-users more powerful than fighters (and, if yes, what level do they take the lead)?
Yes, dabbling into the dark arts make them more powerful, however they can go easily insane. They can lead even at 1st level if they have rolled high Int.
(8). Do you use alignment languages?
I don't use alignment so alignment languages are non-issue.
(9). XP for gold, or XP for objectives (thieves disarming traps, etc...)?
XP for overcoming monsters (killing, tricking them), per miles of explored wilderness and gold spent on carousing.
(10). Which is the best edition; ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E ADD, 2E ADD, 3E ADD, 4E ADD, Next ?
I'd say Swords & Wizardry: Core Rules 4th printing. I have no nostalgia for the previous editions of D&D as most of my rpging occured during the d20 craze and last breaths of 2E ADD.
Bonus Question: Unified XP level tables or individual XP level tables for each class?
Individual XP level tables, dunno why but it seems more appropriate