"Otherwise wonderful parents"...
Here's where I disagree. One parent was never home and had very little idea what was going on in his own home, and the other ignored her child's recurring health issues to focus on a life of junior pageantry and riding on floats in parades.
Personally, I think they were both neglectful, self-centered parents who valued themselves over their own daughter.
Desperate people do desperate things. I don't think they needed to strangle JonBenet - the intruder could have killed her by striking her on the head. I assume the strangling needed to cover up previous injury to the neck, and that's why they felt they needed to tie a cord around her neck. I think the paintbrush injury is meant to raise doubt or misdirect from previous molestation. They had to call police eventually, and the friends provided a buffer as well as contamination. I think they thought JonBenet's body would be found by police before the calls from the 'napper were supposed to come.
I can't see a perp who kidnapped a child from her home taking the risk of being caught returning her for any reason. He could just stop on any lonely road and dump her in a ditch. Why would he take the chance of being caught putting her back in the house? He certainly isn't going to be getting the ransom the RN claims the crime is about.