Regarding magic staves, magic doesn't crit (including Staff of Magus) so crit % only affects your basic physical attack. In other words don't worry about it at all. Furthermore, only staff of magus is based on your weapon's damage so you may as well ignore it, just get the best +stats on the weapon whether it's above or below your level. % chance for debuffs does not apply on spells, only on normal attacks.
I haven't really played with bows, but I don't believe weapon debuffs will affect abilities and I believe as long as an attack is capable of critting any % increase to that will help.
