What an absolute treat of a story— this both excels as a forgery and a complex story "outside" the gameboard, creating some very impressive interplay between both sides of the narrative. I was very positively surprised by the change in tone once you arrive at the first interlude and realize this story isn't just a spruced up EP8-esque purple board, but something more ambitious than this. I love the setting of the forgery forum and the way the protagonist talks about it and its users; while I joined the Umineko fandom at a point in time where these kinds of forum cultures had already more or less started dying, I remember the time I spent lurking there very fondly. (I enjoyed the hints section for that reason quite a bit as well; they were thoughts on mystery/gameboard writing and how they stood in relation to Umineko, after all.)
Big kudos to you for the visual presentation; the use of the box/line to denote story "modes" such as a body discovery, regular gameboard segments or forum discussions are delightful and I must admit that I caught myself gasping in surprise when I first saw the previously black background change to a blood splatter and watched the purple frame turn to the red we're all so familiar with.
I also cannot neglect to mention the amazing answer minigame— as someone who has worked with Ren'py quite a bit before, I was very impressed by your system and all its quirks, and I do admit that I ended up going through the code just to see what else I could find in there. (It did not disappoint. The poisoned sugar entry was hilarious.)
In regards to the mystery itself, of course, I was very impressed with how it was designed; really throwing everything at you without mercy, making you doubt yourself and your refusal to believe that these incidents just being entirely impossible— a seemingly ruthless attack you can't reason your way out of.
I don't know to what degree you planned this, but to me, a lot of the gameboard's workings felt like a cleverly devised trap to trick you into gaining tunnel vision and only focusing on the wrong parts, especially for people who've consumed Umineko fanworks before, who are already familiar with the tropes and tricks of the genre and naturally hold certain expectations when reading a story like this. From the cast (and survivor) choices, the character writing, the chaotic pacing and hectic back and forths between the survivors during key moments, to the clarification of the rules to lull you into a false sense of security regarding certain culprit possibilities, the twist of the second narrative that's correlated with the one on the board... every moment felt like it was designed to throw you off and cause you to start second-guessing literally anything because you could no longer be certain if it really did happen the way you assumed it to be. I had to sleep on the mystery to really gain the clarity to get the correct approach, which I happened to find through the final incident; though the other murders still gave me a lot more trouble and kept me doubting my approach again and again until I finally got it right.
Admittedly, I felt that the second incident wasn't exactly the "weak link" of the mystery that the hints implied it was; if you solve it, the rest of the mystery would most likely be very easy, but I found it to be a much more difficult starting point than the third incident. I personally think that the "lucky" factor relating to the door wasn't exactly properly hinted at, which led to me just randomly picking all the door options during the minigame because to me, it seemed there wasn't really enough information for me to go off of, as well as the possibility it wasn't even locked during the incident to begin with. This is, of course, just my opinion, and I could've overlooked something in the narration or purples pointing towards that, but I wanted to share this with you all the same.
Overall, I have to say that I absolutely love this game. It's well-written, has a challenging mystery, a stunning sense of style and visuals, and a top-notch way to put your skills to the test. Thank you for sharing your wonderful work, and I hope this comment was, despite its wordiness, able to communicate my appreciation. Have a nice day or evening!