I think it looks a lot like Seraphina tried to ambush Nanami only to be interrupted by Zack. Nanami seems--seemed--to be really interested in solving mysteries, so pretending to have found a clue would be a good way to lure her out. It would explain why the dollhouse was dark, and maybe also why Zack said Nanami was tied up.

However, for Seraphina to be the final culprit, she would have to have Tamamo's mirror--presumably just to implicate Tamamo, because she wouldn't know anyone would end up needing healing--know how to use it, be in a condition to use it after Zack's attack, and have time to get the rope and the mirror away from the scene after the attack and get back in time to fake unconsciousness when the body was found. So it's not the simplest solution.