I'm with Lukas...you guys are making this FAR too complicated. You're bastardizing a game that's already bastardized from a card game! lol
Anyways, if it's too difficult for the traitor (which, I'll have you know, is NOT a fact yet as we haven't even gotten far into a single game, let alone finished one), then the easiest thing to do is to change the victory condition for the traitor to only have to eliminate the rebels, knights, and king (king MUST be last), but NOT the other traitors. That way, on larger maps with more than one traitor, it's essentially another "team" like the Rebels have.
As for changing the victory conditions for the Rebels and Knights per Butterhead's suggestion...the entire point of this style of play is for one side to protect the King and the other to kill him. Changing it so that the game goes on after the king is dead fundamentally changes it. If the traitor(s) can't help the Knights protect the King until the Rebels are weakened, then why should the traitor(s) have a chance to win once the King is dead? They failed to accomplish their objectives. Let's not baby the game and turn it into a little league game where everyone gets a trophy and gets multiple chances to "win". Accomplish your objective or lose. If it's too tough, keep practicing and get better. Don't change the game.