Ames, I used to think like you, that PR did it and 'blackmailed' JR into helping with the cover up because she knew about the molestation.
But there's a bugaboo (if you'll forgive the word). It appears that the sexual wound inflicted on JBR was inflicted PRIOR to JBR's death--because it bled, which it wouldn't have done if it had been inflicted after she was dead.
There are only two possible explanations for that wound. One, JBR really was killed by a sex-crazed pineapple feeding wandering pedophile who was really bad at kidnapping but really good at leaving no direct evidence of his presence, or two, the wound was inflicted to HIDE evidence of prior molestation.
But that means whoever inflicted it KNEW that JBR was being molested, and definitely WANTED to hide/confuse the evidence of that prior molestation!
So PR would not, IMO, have inflicted this wound on her still-living daughter to destroy or seriously compromise evidence of prior molestation and THEN gone to JR and said, essentially, "Help me or else."
So, either PR is herself the molester AND killer, or JR is the molester AND killer. (Or, to be fair, the wandering pedophile is the molester/killer).
But if PR is the molester/killer, how does she get JR to help her cover up the crime? Whatever 'dirt' she might have on him, it's not going to be the same as a murder rap.
No matter how hard I try, I can't think of a situation where PR could trick JR into helping her cover up the crime. But I can think of a situation JR could have contrived that might, possibly, have worked to get PR to help him cover up the crime. There's even some physical evidence that can be interpreted that way, though I'm still working it all out.