Any such accidental head injury would have some corresponding item in the house that had caused it: a wall, table-edge, bannister, something. That object would bear trace evidence as well as a sinister shape.

There was no 'accidental injury' to the head, there was a carefully aimed blow that delivered a fairly major force. I would NOT refer to it as a 'coup de grace' because no merciful ending of pain was intended, it was more a final 'take that, just to be sure'.

The problem many of us have is that we don't know if the person who delivered that forceful blow did so: to get rid of a witness to his perversion because JonBenet knew his identity, to get rid of a possible witness to his perversion because JonBenet might eventually describe and later identify him, to inflict pain on the family, to give him pleasure?