How does % less projectile damage from support items effect ele damage bow skills?

I've looked everywhere and couldn't find the specific answer to my question when it comes to a certain build.

I and using galvanic arrow and have massive elemental boosts in my build but want to try out arrow nova and possibly a volley support. does less projectile damage effect the ele damage my build does or is it for the actual arrows that my bow shoots?

Also with barrage support do the extra arrows it shoots hit a single target boss? I know that a single target cannot be hit more than once in the same attack. So if I'm shooting 3 arrows in one attack will the arrows beyond the first split or pass through them?