I have been to 781 rooms. Unexplored to me is a spiky area in the chapel blocked by a magic door which I can't open in either bat or mist form, and a one-way wooden panel in the caves with some nice powerups in it which leads from an unexplored room.
I can get the bad ending whenever I want by battling Richter but I feel I need to get Maria to smack some sense into him. I have tried using the gold ring and clock in various rooms, including in the clock room, and I tried various things in the Doppelganger room since its door has a design I recognize from Harmony of Dissonance.
What am I missing?