Couldn't the crime itself have happened in the cellar, and some of what we see as staging was simply part of the crime as a whole?
The most curious part of this case, IMO, is the use of a garrote. If I'm going to believe there was rationale enough to stage the crime scene immediately following the death of their child, than I find no rationale in using a garrote for staging.
I think the garrote might have already been in place when she was hit over the head. I don't think it was tight enough to asphyxiate her yet. I think THAT happened later, during the staging process.
I think it's plausible that JB wet her bed, changed her clothing, went downstairs, and ate an uneaten piece of pineapple from BR's abandoned bowl. JR at some point got up and brought JB down to the cellar, where he sexually abused her with the paintbrush, fashioned the garrote because he had a kinky taste for it (not tight enough to show marks/choke her, but maybe just for simulation).
She was uncooperative and noisy, he hit her over the head, and once he realized how badly she was hurt, he went to get PR. She knew all along about his abuse, so she had to go along with it.
She wiped JB down, put her pants back on, maybe moved her. Is that when the garrote got tightened? Then they hid any obvious evidence and PR wrote the RN.
IMO, this theory doesn't cover all the bases, not by any means. But to me it is no less believable than a nine year old doing it, or a split personality. JMO