I'm not really a huge fan of the mass council rooms, especially since the only ways to gain actions are your one university, and the only way to break even is apprentice/golem into apprentice. I get the synergy with possession, but that's a lot of moving parts that you don't really have a good way of putting together.
Did not see that Possession-synergy, but I think that it cuts both ways. If they possess you they can boost their own next hand with the Council Rooms. I think you're better off making your deck less appealing to Possession strategies.
Well, if you expect to be able to play a possession every turn, that could actually be bad for your opponent. They play, say, 2 council rooms on your turn, go up to 7 cards, then you play one and a possession and you suddenly get to possess an 8 card hand.
It seems like a pretty fragile synergy though, and I don't think decks on this board can consistently play possession without university/apprentice, so I don't think it's worth it here.