Yeah, if a dragon breathes fire, I think it has at least a 25% resistance to fire, and the same goes if it breaths ice/frost. So you hit them with the opposite element to inflict extra damage; they take like 25% more damage from the opposite element of what they breath.
Yep. Until I learned "dragonrend", I carried a frost bow and also a fire bow, and pretty much only used them on dragons.

