With all due respect you haven't read the book. What the heck would Patsy do with a brain damaged JonBenet? IMO Patsy would rather see her dead than damaged in some way. IMO I've often considered that perhaps Patsy caught them ...yet again, engaging in inappropriate behavior and being stressed out by the last minute packing, snacking, getting kids to bed while John was of no help, simply went to bed..... it was the straw that broke the camels back and she swung in a momentary fit of rage. Patsy would have heard her skull crack and witnessed the immediate result.
Do you believe an intruder killed JonBenet?
BBM. Your theory may very be what happened, imo.