These questions are good because Adventure Fantasy Game is mechanically well different from D&D despite being essentially the same game. And almost completely stat-compatible. Grab the free AFG Lite PDF here, full chargen included. Buy it here.
(1). Race (Elf, Dwarf, Halfling) as a class? Yes or no?
Possibly. AFG allows to pick a different class (actually, a Way, which is a bit different as it it represent focused growth in a specific discipline) every time PCs level up. Demihumans have special classes of their own that they can take.
(2). Do demi-humans have souls?
Yes, except elves.
(3). Ascending or descending armor class?
Armour reduces damage. In the lite version, ascending, kinda.
(4). Demi-human level limits?
All PCs stop levelling at level 12, 13 with a trick, but they can get other perks.
(5). Should thief be a class?
There are Practitioners that gives you experience with a specific task.
(6). Do characters get non-weapon skills?
Yes, with the 5MORE system. More commentary.
(7). Are magic-users more powerful than fighters (and, if yes, what level do they take the lead)?
Fighters are bread, Casters are Nutella. They’re different. Fighters kick so much ass in melee it’s unreal. Caster change the laws of physics.
(8). Do you use alignment languages?
No. Languages are national shibboleths though.
(9). XP for gold, or XP for objectives (thieves disarming traps, etc…)?
Levels and perks for accomplishments and looting important treasure troves. There are no equivalent for D&D XPs.
(10). Which is the best edition; ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E ADD, 2E ADD, 3E ADD, 4E ADD, Next ?
I roll with S&W Whitebox. Sometimes Moldvay/Mentzer/RC, which are for me equivalent.