[Draggor]

1. answered above. Note that having keen and improved critical only triples a threat range, multiples of the same type stack. So a keen long sword with improved critical would threat on 15-20.
2. Because magic in general has been depowered, so spells that were weak before are now powerful relatively speaking. The new dominate monster lasts a lot longer (at least under pen and paper) than the old domination did.
3.NWN is set in the year 1372 by the dale reckoning. Near the beginning I'd say. BG finished at the beginning of the year 1370, how far in depending on how long you took.
4. Yes you can. I soloed the whole game with a fighter/rogue.