While playing tonight I have a (Blue B-side) Mage sitting in the council chamber [no more than 1 Mage per player in this room] and my opponent has a Mage in the same room. I currently hold the level 3 spell (possession) [action: swap ownership badges with between to mages on the board. Their ownership is permanently swapped] and my opponent has and Orange Mage (one I could really use) on the archmages staff (a spot I really need). What happens when I activate my possession spell swapping ownership of my blue Mage and her orange Mage? (In other words when she suddenly has two Mages in the council chamber)
For Council Chamber A, nothing breaks the one-mage-per-player rule. If you already had a mage in there, an opponent's mage in the Council Chamber would be an invalid target for Possession.
I don't prefer to play A, personally. I'm all about the bloodbath that is Council Chamber B.