Yo, I already finished reading the part on Wednesday because, as I wrote in the last week’s thread, the story started to flow nicely (Except for 月夜がまくしたてる). Had to look for something else to read in the meantime to not get ahead too far.
Let’s see if I can still manage to organize all my thoughts.
First, @simias was right on the money with his assumption that they would think it is a staged suicide. Well done 
The deduction from the dying message with chocolate to it being poison was crazy. It sometimes feels like I’ve read too few mystery novels for the mysteries in this book to be solvable by me ^^.
At first, I thought the story would go in the direction of 遊馬 being a failed protagonist that slowly spirals into mass murder. First, when he didn’t dispose of the poison and then when 月夜 came to talk to him. But alas, the story is really picking up steam with a murder, we as the reader know nothing about.
I suppose 老田 probably was seen in the morning before starting to set up the table, so the murder would have been committed in the morning, and not at night. I can’t see any motive for the murder yet. But my prime suspect would be 月夜, she seems the smartest to me.
It’s another closed room mystery. The fire could have started because of some lensing effect with the bright light in the dining room in the morning. Given the door locking mechanism picture (which I loved, to check I understood correctly), I also think it would be possible to lock it from the outside with a thread or something similar.
Don’t see the relevance of the blood message on the table yet.
One thing I’m curious about is the constant reminder of the amount of revolution he’s walking up and down in the spiral stairs to get to the rooms. Half expecting there to be some kind of clue later on where we find out the rooms can move or something 
I’m really looking forward to the next week to see what clues the author gives us. And also love to hear any theories from you.