[Azule]

The problem with this theory is how Erika's red is a contradiction with the other red.
Its never mentioned which one it is, be it there are 18 "people" or 18 bodies.
This also means there could be 17 "people" or 17 bodies.
If we take Erika's to mean 18 "bodies" then its correct, and if we take Beatrice and Battler's red to mean 17 people, then the whole timing issue is solved. But then that leads to the whole closed room guesthouse situation- Erika couldn't figure out how Kanon left the guestroom, meaning he either was never there, or there's a way out which isnt through the seals.
However, if Beatrice and Battler's red means 17 bodies, then ShKanon can't work out without a visable way to exit the closed rooms which are confirmed sealed