I wouldn't even allow swapping counties between players, you can always dow each other and swap the land in a mock war. If you want to show your vassals they don't know whome you are going to sell next, take the BB hit for doing it.
Alternatively editing the counties should IMo also include the Bad Boy hit to both kings.
Generally editing mid-game is in my opinion not good, unless it is to do something that can't be done in game. Swapping counties can be done ingame, killing off vassals can be done ingame. If you don't want to take the prestige or BB hits for it, then don't do it.