1. Suppose there was one of these low tech curved metal gadgets, keys we used to call them, overrides the number code pad, when you go out of the cockpit you take it with you. Having a system which can, through malfunction or otherwise, lock people out of the cockpit was always going to cause grief one way or another.
2. Suggesting having an FA in the cockpit monitoring the remaining pilot is dumb. They have no idea what is going on and most likely couldn't do anything about it if they did.
3. If this was deliberate, i.e. the co pilot was not in some way incapacitated, then it wouldn't have happened with the U.S. 1500 hour system. He would still have been flying a 402 at this point. If that.