Mithril does affect quest success because it affects your stats directly. I'm not sure if this got fixed but spells used to carry through resets while mith wouldn't, so someone resetting could use a bunch of spells before the fact and get a dozen or so quests done. Sweet stuff. No clue if all that's been changed.
Also, (I'll make an op edit later when I get around to pc)
******NOTE******
The nob/xtal drop rates are possibly inaccurate. I have reason to suspect the devs changed the rate/s since I last tested.