You dismiss the idea of sky marshalls because you think that one hijacker could create a disturbance, forcing the marshall to reveal his identity. But so what if he reveals his identity. He (presumably) has guns and martial arts training, he’ll be able to take on four cardboard-cutter weilding hijackers.
Plus, it’s not clear that an “air rage” incident would force the marshall to reveal himself. In most of those incidents, several passengers usually tackle the person, so it wouldn’t be clear which one was the marshall.