So, now that Novel #5 is over, let's think back about the "real world" events : who killed the seven victims of this iteration ? We'll assume, of course, that it's not Natsuhi.
I don't think it's Battler either, though he might have been a partial accomplice.
I absolutely love the idea that Rosa and the cousins somehow faked their deaths (as Erika didn't bother with checking the "corpses" thoroughly, and their deaths were only confirmed at 24:00), though I'm still a bit hazy about how exactly Battler argued that. Let's assume for now that it worked. They'd have no trouble "moving" the fake bodies while everyone is checking on Genji, of course.
Of course, Battler must have been in on their plan. And there's absolutely no problem with that : a very early scene I haven't mentioned indicates that Battler had renewed contact with Jessica by phone just before Natsuhi receives her first call from the mysterious boy from 19 years ago. Which makes me think that the cousins and/or Battler are also behind those calls, by the way.
Which one of them is a more dicey question. Genji's obviously an accomplice, so the calls could easily be coming from inside the island. Actually, considering what we know about the communications, the midnight and morning calls must have come from inside the island. And we know that the cousins' room has a phone. Battler seems to have an ironclad alibi for the midnight call... except that a good part of the scene just before discusses the presence of tape recorders/players among Grampa's possessions. Chekhov's Gun at its finest, ladies and gentlemen.
The more I think back about the mystery, the more I notice that every single apparently trivial detail of this chapter was an important clue. (Or a red herring.) Wow.
To conclude the theory : the cousins are playing a prank on Natsuhi and everyone for whatever reason (a game to entertain Maria ?). One of them might be the kid from 19 years ago (my instincts are on George, for some reason), and have actual bloodthirsty motives. That one kills the others. They also kill Hideyoshi later, having probably provided him with instructions to go there like Natsuhi.
For Genji, my money's on Kanon doing the deed (though I have no clue why). As for who killed the mysterious culprit later, I have no clue.
This theory is full of holes, but it's certainly a lot more interesting than "Natsuhi did it", isn't it ?
