A10: There's nothing to stop you from taking Improved Critical (longbow), AFAIK. Likewise, you can give it the keen enhancement (it's a piercing weapon; there's nothing to limit it to melee weapons), and the keen edge spell specifically says "Target: One weapon or fifty projectiles...." Beyond that, I don't know of anything.
Q25: If I'm a cleric 3/paladin 5, what's my effective cleric level for turning undead? Is it cleric levels plus paladin levels minus 3 (therefore, in this case, 5), or cleric levels plus paladin levels (8), or do the two not actually stack at all (so take the higher one, which is cleric 3)? I'm guessing it's the first one, but I'm not sure...