This was soooo fantastic! I've just recently got into/finished watching a playthrough of Umineko, so this was fun and felt like an authentic bonus minisode. Getting to play the purple truth minigame for myself was a blast, and I loved the crime reconstruction aspect of the locked room circle. I did end up tunneling on (SPOILERS)
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Battler, as I figured the trick for the boat killing was something as hackneyed as "everyone else behind Shannon and ahead of Battler was asleep as he stood up and made his way to her" LOL. (BTW, the "type-your-own solution" section for this made me grin ear-to-ear as the solution dawned on me once I actually looked at the diagram and remembered we were working with a 3D space) I ended up very attached to him being the culprit, and came up with a solution I think works for how he could've done the locked room, if he had been the murderer?
He locks the main hall-dining room door, tosses its key into game room, locks game room, tosses game room key into dining room, locks kitchen-dining room door, opens window in servant room, puts the kitchen-dining room key in there, locks the servant room door and simply palms the servant room key for now.
All doors are locked, but the trick was when the survivors gather in the main hall in the morning and Battler relents on breaking down the door, opting instead to check, rather than going to peer through the dining room window, what he REALLY did was enter the servant room through the open window, grab the kitchen-dining room key, lock the window, unlocks the servant room door, goes to unlock the kitchen-dining room door, deposits the servant key, relocks the kitchen-dining room door from the outside, heads back to the servant room, deposits the kitchen-dining room key there, then rejoins the group to bust down the main hall-dining room door.
At that point, he makes sure to grab the servant and game room keys, and I read his hesitation in opening the servant room door as him feigning apprehension while he covertly locked the door with the key before passing it to Erika, who confirmed it was locked, because it really was. So the trick was that it was locked when Shannon checked it in the morning, as well as when Erika checked it after breaching the dining hall, it just wasn't locked between the two points. I was right about a door unlocking being pantomimed, just whiffed on which one/who did the gimmick, lol.
The Nanjo-Erika conversation about being discreet in what you share was excellent foreshadowing for who attacks Shannon on the boat and why, btw! Should've immediately tipped me off about who the culprit was.
I loved reading through the hints, more so because it felt like talking with a friend about Umineko and impossible crime novels in general!
Again, really great job, and thank you for the fantastic and free game: I'm currently working on my own murder-mystery visual novel about impossible crimes with some Umineko flair, and I'm so happy to stumble across your work! (Someone had suggested Eulogy for Reason as I was looking for taped-room mysteries, and I was ecstatic to see Umineko fangames also listed here!) Can't wait to work through the rest of your stuff