Ok here it goes
1) Sneak attacks occur when your opponents are caught flat footed, but i don't really know how it works, it is again been calculated by the game.
2) Any magical weapon can hit wraiths or shadows but they don't always do damage.
3) I don't remember Jarial's stat but i guess he could be multi to a fighter or cleric.
4) These spells require you to also make a successful hit when attacking in order for the spell to be cast otherwise the spell is wasted.
5) No
6) These skills are used automatically, for example if you near to an opponent the character throughs a dice according to his skill of move silently and if you fails the opponent will notice him sooner and might have the advantage to attack first.