First of all I'm pretty certain Rena+Mion+etc. actively tried to keep quiet to Keiichi about the barabara murders.
Rena's first changed scared the sheets out of me
but it suggested to me that at least in Rena's case Rena doesn't seem to have an active acknowledgment of what changes occur to her.
The subsequent occurences however made me consider some other possibilities. Either the "bu" is actively a part of the "cover-up" (which could be suggested from their non-acknowledgment of the barabara thing), or they seem to be controled by something outside of their normal will. It is like they know they are controlled yet are already active participants. They've given up their wills? There is some sort of control factor among the "changed" girls that doesn't seem to be very obvious. Some psychological problem doesn't seem likely (especially if its prevalent among the townsfolk). Oyashirousama? I personally don't think so.
Jirou's death struck me as bizarre, however the later occurences suggest that the suspected 1-4+~ suspects could have honestly been anybody, even if what Mion said during some of the portions is true. I feel this way because Mion only said Keiichi would taste the same fate as Jirou, which doesn't imply that she did it herself. It seems from what Mion was trying to (inject?) to Keiichi in these portions was probably something Jirou couldn't avoid. Is this particular thing the "madness" that killed Jirou too? Probably. The fact that the note Keiichi wrote was tampered with suggests this highly.
I'm sure I have more thoughts but...
Uhhh, that nurse that disappeared (Takano?). I find her suspicious as well (some of her lines were creepy), though she disappeared that doesn't mean she died too or anything. The Onikakushi itself might just be a way of hiding members of the suspect group, or people who know to much.
Finally, what Keiichi said last over the phone, that he wanted to think humans were behind the events. This part I probably need to look over again, but I have a feeling Keiichi's paranoia took over in a sense... and metaphorically, "I did want to think that humans were the suspects of the events, but there is oyashirosama..." a statement about the barbarity of humans? I also have a feeling Keiichi ended up suffering during his last moments from some nasty illusions. Something similar to Jirou's last moments?