Aren't most people fired for just cause? Also, I always that most states are at-will states, which means they can fire you for any reason at all.
I always thought you could file unemployment only if you are let go/fired, but not if you quit. Maybe they are hoping that you quit. DO NOT.
Also, if he knew you had access to his email, what's the big deal? He must have some shit on their that he wondered if you saw. You should say nobody ever told you that you could not have personal access to his emails, that you dont have time to read themand clicked on it by accident. Stick to whatever story you had, and don't waiver.
Show up Monday with your head held high.