Crime scene in the servant room

Axioms

There are no hidden doors in this room.
There is no way to get in or out other then the door and the windows.
'There are no hidden doors'
What about the original key to this room, the 'keys to the servants' room'?!
Indeed. Those are kept in the key box in the center of the servant room. There are several servant room keys, but all of those are inside the key box.
Entry and exit are impossible except for the single door and the single window.
And those were both locked. The door and the window do not permit any kind of entry or exit when they are locked.
It is impossible to unlock the door without a servant room key or the master key.
No one exists in this room except your group. 'Your group' refers to Battler, George, Maria, Rosa, Genji, Gohda, and Shannon.
There are only five master keys. There are only five master keys.
I keep my promises If you solve the riddle of the epitaph, you should be able to reach the Golden Land. When you do, the ceremony will end. No more people will die.
They would never mistakenly think any other person was Kanon!
No one else can go by Kanon's name! A different person can't claim that as their name!

Corollaries

The scene in the servant room is false retelling

There was no zombie Kanon. Kumasawa and Nanjo are killed by Genji and whoever is Shannon at the moment and the bodies are carried away. Then Gohda is threatened so that he has to play along with the absurd story. Rosa knows it's fake too, her job is just to make accusations and separate the group.

The scene in the servant room is an elaborate staging

This is the version where we assume Kanon was there. In that case it was Sayo as Kanon, X as Shannon and everyone else except Gohda staging a shocking scene for the sake of it. Nanjo and Kumasawa are not actually dead and are killed later during the murders in Natsuhi's room.

What made it extra shocking and hard to talk about for Gohda was Kanon and Shannon suddenly being in the same room even though he knows they're supposed to be the same person.