If you mean she could have been dropped on a line and the head THEN hit a solid object (maybe the metal structure of the staircase), they yes, I could see that. The problem is that we're dealing with something rare as it is, so I wouldn't add even more unlikely elements to it. The simplest explanation is that someone threw her very hard, she hit something fairly soft and her head snapped back against something really hard. But you may be right, it could have happened as a result of the neck injury if the conditions were JUST right.
When I read all this I finally realized why the evidence was pointing everywhere and nowhere. In a way, that fact alone points to a freak, or rare, accident.
It would also explain why Karr said "it was an accident".
Doesn't Karr say somewhere that he was sitting or playing with her on the stair(s)?