"Doesn't Karr say somewhere that he was sitting or playing with her on the stair(s)?"
Could be. I think the more salient point about this is that you already have a statistically rare event in the home that night - the manner of head trauma. To add an intruder seems to be pushing the odds to absurdity. Figure the odds on a head injury accident like that on the SAME night an intruder breaks in...He couldn't have done it intentionally b/c it's not the kind of wound that you can plan, it just rarely happens.
If there is something about the pathologists reports themselves that might explain it, then I think that's a valid point. But we'd need some experts to speak directly to this issue of biomechanics to answer that.
Sitting with her on the stairs refers to his story that he was at a party at the Ramseys on the 23 of Dec. at which he showed her coin tricks and gained her trust. It isn't refering to the night of the murder.