It's been 24hours, so we can leave the spoiler tags aside for ch15.
Actually, I don't find it too abstract that Mitsuki's father is involved. Remember the first chapter, Mitsuki and Yuu met because she was shopping with "Papa". She passed on a test so "Papa" wanted to buy her whatever she liked. And she also seemed like quite the Daddy's Girl seeing how she, as a policemans daughter, reacted on the underage smoking. I also remember that I found it weird that the translation said "I'm shopping" and not "I'm about to go shopping" which would indicate that she waited on her father but instead it sounded as if she was already doing it. But where was her father? Mitsuki just left with the others for Karaoke but I can't remember that she ever called him to stall the shopping (plans).
Personally I find this development more existing than having Mitsuki alone or one of the other kids as the culprint. It's still hard to believe that a teenager could built up the whole thing (the place, the game, the background researchs). The situation looks completely different with an adult, and a cop to boot, in the background.
I mean, how could Mitsuki be related to the others to such an extend that she wants to kill them off? It sounds more understandable to me that her father was the police officer in charge of all their cases but couldn't touch them (covered up by higher ups, blackmail, ect) and therefor developed a psychotic grudge against them. I'm very interested to see how Yuu is involved in this. Yuu forgot something important about "tomorrow" in the first chapter, which gave Mitsuki a sad look on the face (her birthday or some memorial date, I assume), maybe that was a hint as well. What kind of date could it be other than a birthday? Maybe the day her mother died? Seeing that Mitsuki and her Dad aren't right in their minds anymore, there must be trigger for both of them to act this way. And maybe Yuu would be (accidently) involved in this? Ah, I love weird theories at 3am ... *goes to bed* >.>