It's already working for the demons. The fact the gods don't anymore have one of the keys but it's in the same world as Bahamut is weakening the seal so much that the gods' army is basically tied to containing the monster. So, the demons were able to, for example, attack the human city at leisure, save for Jeanne. I imagine the gods would have sent some reinforcements under normal circumstances. So, it's not entirely an all or nothing situation.

I liked this episode well enough. I'm not sure what you guys' problem with it was. The massive battle was very nice. Reminded me of the one in Return of the King.