The shields absorb damage regardless of the damage type.
No, it adds +50% Fire resistance and -25% water resistance
I know what the description says about the spell, but that is not how the shield spells work. In addition to the listed effect, those spells give you an additional health bar.
Yep and the additional health is based on the current vitality of the target. I think the air and water shield are the most powerful shields, because the target can not be stunned or freezed, too. With these shields Astarte can not be killed by the enemies and you can focus the attacks on the dragon.