I'm answering them based on my homebrew campaign setting, Domikka.
(1). Race (Elf, Dwarf, Halfling) as a class? Yes or no?
No. There are no elves. I have halflings in an unpublished story, but they get slaughtered. Dwarves are evil. All of them. They like to hunt humans for their hearts.
(2). Do demi-humans have souls?
Demi-humans? You mean Goblins of the Woods or Demons of the Earth? Heck, no. That's why its okay to kill them.
(3). Ascending or descending armor class?
Ascending. The days of Thac0, alas, are over.
(4). Demi-human level limits?
Goblins of the Woods and Demons of the Earth have levels?
(5). Should thief be a class?
(6). Do characters get non-weapon skills?
Only if they ask.
(7). Are magic-users more powerful than fighters (and, if yes, what level do they take the lead)?
It depends on whenever the player decides on having his character take the lead.
(8). Do you use alignment languages?
(9). XP for gold, or XP for objectives (thieves disarming traps, etc...)?
Both. But I'm thinking about switching to the DCC RPG experience point system.
(10). Which is the best edition; ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E ADD, 2E ADD, 3E ADD, 4E ADD, Next ?
Sigh. In the grim darkness of the near present there is only Edition Wars.
Bonus Question: Unified XP level tables or individual XP level tables for each class?
Depends on the edition.