I still think Patsy inflicted the head wound in a fit of rage and John helped with the staging, partially to cover evidence of his own previous sexual abuse of JBR. And that's why they stuck together after the fact: both had a hand in horrific crimes they did not want the world to know about. Child abuse spun out of control is, to me, the simplest and most likely explanation here. It happens, even in wealthy/educated/high-profile families. Even the closest friends and family don't always know it's happening. You NEVER know the dynamic of a household unless it's your own.
It's not uncommon for spouses to stick by the abusers rather than protecting their children. If both parents are abusive--one sexually and the other psychologically/physically--the child doesn't stand a chance.
If I'm ever proven wrong I'll gladly apologize to the R's myself. These are just my impressions based on everything I've read/watched/heard/seen over the years. I still think Burke was a victim more than an offender, but am open to other possibilities.