I do the rule where direct spells do base damage and the caster can choose to add net hits to the damage but each one added raises the DV by 1. This balances the insane power of direct combat spells in my opinion. Yancy chose not to add hits because of the added drain.
yeah, i'll say full cover protects against flashbangs. If you're not fully covered and out of site i'll just assume with your amped reflexes you used a free action from your 3rd IP as soon as you saw the grenade to duck and be completely under full cover.