Damn, I loved this. Excellent job. Umineko is one of my favorite stories and this is such an amazing tribute to it.
SPOILERS
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I love the idea of using an Umineko gameboard and forum as means to reveal another murderer. I've lurked in a few Umineko forums and this idea just is interesting. How curious that the true culprit must have been obsessed with Umineko and fans' gameboards. Maybe they told Tohya the truth because secretly they wanted to be found out all along and saw in them a real detective who would really understand them? Was their Umineko obsession an unconscious cry for help for someone to reveal their own crimes? Hm.
Lastly, it's late here and perhaps I misread but did the culprit outright kill both their mother and grandmother, or was it mother and grandfather? I think I read it was the former but then why did they kill the grandmother? Did I miss something and she was also complicit in covering up everything?