It is possible that the head wound occurred while the child was being strangled if she was struggling violently and/or if the killer was in some way tossing her around violently while tightening the garott. Both the strangulation and the head trauma were fatal in and of themselves. She clearly survived the head trauma for at least a short while after it occurred. Similarly, with the strangulation, there are clear signs she was alive when it occurred.
If I understand the autopsy conclusions correctly, the head wound had to occur either shortly before she was strangled or during the strangulation, but before her heart stopped pumping. I don't think it's possible, lacking other evidence, to conclude precisely in what sequence the head and neck traumas occurred, which is probably why the coroner was vague on this point.
Medical professionals who study the autopsy photos and descriptions could give us an estimate of how long she might have survived such severe head trauma without medical support. My guess is a very few minutes, at best. My guess is not long enough to carry her all the way down to the basement if she was struck on the head in her bedroom or the kitchen. If I'm right, this means the two fatal events did not occurr in different locations.
The autopsy says the whole inner surface of her scalp, her skull and the right side of her brain were coated in blood. So her heart did keep pumping for at least a brief period after the head wound occurred.
Because there is also evidence of the child being alive when she was strangled, the only logical conclusion I can draw is that the head trauma occurred either a very short time before she was strangled, or during the strangulation.
I want to picture the scene as someone hitting her from behind and then doing the rest, because I want to see it as the child mercifully never knowing who and what hit her.
BTW, even though no evidence so far tells me Karr really is the killer, I can see how some twisted SOB like him might think JonBenet's death was an accident. The head wound may very well have been an accident. The garrotting was obviously not an accident, and after reading the autopsy report and seeing the photos, I'm no longer convinced it was done to cover up what someone thought was the child's death due to the blow to her head.
Your evaluation of the head trauma and strangulatioin is the exact opposite of everthing I've read and heard.