Missile Weapons and Frigates

And does accuracy above 100% help at all? For example, can it negate some amount of Evasion of the target?

Frigates are meant to be anti-fighter vessels, and I don't think that it's a coincidence that they get +50% accuracy while fighters get +50% evasion. In GC3, at least, accuracy adds to hit chance while evasion subtracts from it, so they would equal out when a frigate attacks a fighter, and yes, a frigate would effectively have a 100% hit chance against other vessels unless those ships have their own source of evasion. (Higher than 100% wouldn't do anything except help to cancel out further evasion bonuses.)