Well if the disobey the Captain's order to be seated then the alleged Air Marshalls have the right to seat them. When you are on an Airliner the Captain is the law.
Try it sometime and see.
there are 14 terrorists on the plane, they don't want to hijack it, they want to blow it up. if two of them pose as the disruptors, as soon as the marshalls approach them, some of the other members of the team use that distraction to set off the bomb.
there is no way to stop an attack like this once you allow the possible terrorists to board the plane. none.