yeah bell is for the fusions, i think theyr pretty cool (especially futomimi). You'll find beelzebub, you need to enter a hall at a certain moon phase to be teleported where is. Npcs will refer to him as the witch king or something.
No, you only need to go through the special door during the NEW Kagutsuchi phase. Then make your way north through a large cursed area. There's one of the doctor spirits near the door though.
Anyway, to get the bell speak to both of the specters in the one area of the Kalpa, go to the cemetary where you emerged from the tunnel in Ginza to collect it, then speak to the two spirits again to unlock the fusion.
What is a TUNA!? A miserable little pile of secrets. I HATE Grenchlers.....