What an amazing mystery to solve! I loved all the small changes made to the gameboard compared to original (Erika being a lawyer, Shannon rejecting George, etc), giving the reader a very fresh experience but still oddly natural. The intermissions with the online message board was also a super cool idea! So many funny references, memes and slang making it feel very authentic. (I think the only part I didn't like so much was the meta-mystery of who killed Tohya. It got kinda confusing, especially the ending...)
Despite a lot of struggling, I didn't solve the mystery without using all of the hints (SPOILERS BELOW)
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First I believed that Kinzou was the only possible culprit of the Locked Room Loop. My idea is that Kinzou left the window in the servants room open, then after the group discovered the bodies in the dining room Kinzo quietly slipped away from the group and entered the servants room through the window, locked the window from the inside, and hid inside the room until the door was unlocked by Battler.
Kinzou could have killed Shannon the same way that Beatrice did, by crawling beneath the chairs. But I also had a different theory; Battler's purple was that anyone standing up from their seat would have been spotted, but perhaps Kinzou was never in his seat - instead using a fake dressed-up doll or something. It would have been easy for Kinzou, since he ordered the boat. And Kinzou is the only person that could have killed Shannon through the window.
But alas, I couldn't find a way for Kinzou to kill Nanjou. Only Erika (by using something other than a gun) or Beatrice (by Nanjou not dying instantly) could have killed Nanjo.
It was only after reading the last hint, that I understood Beatrice must be the culprit. Erika's purple had tripped me up:
"the doors to the dining room, the door to the servant's room and the door to the game room were all locked with their respective keys, of which there was only one per door."
"Beatrice unlocked the door to the game room, and it was definitely locked before she did so."
... I suppose Beatrice locking and then unlocking the door to the game room does technically fullfill the purple