I don't think I checked mana usage on any warrior skills, but I assume it does. It cuts the mana needed to summon your creature.
I used Whirlwind a bit; never tried Confusion.
Destruction just works on magic spells, AFAIK.
The best resistance to have is whatever your opponent is currently attacking you with. I kept them relatively even, but at various points in the game one could be significantly higher or lower than the others, depending on equipment I found, etc.
I kept strength and dexterity relatively even. I boosted intelligence for the magic resistance (and indomitable will), but my next character I'm going to see if I can get away with neglecting intelligence.
