How do we know she wasn't just hit by a car, or even a truck, that could have been higher and struck her head?
Hey, if this person was worried about vehicular homicide, I could see them wanting to move the body and take off without reporting it.
If it being in the middle of or early in the afternoon is an issue because this is likely to have been seen, maybe they put the body in their car/truck asap after realizing she was dead/dying and then brought it back there after. Where else would they put it? If it was put somewhere else, might be likely traced to them. The time of day and the not being seen doing any of this, does seem unlikely, even though someone said the traffic would be low at this time of day, but I doubt it would be so low as to not be seen at all.
Anyways, I wondered also about this theory, because of the single blow to head that killed her. The other brusing to head, and scratches on wrists, other bruises elsewhere, could all be consistent with a car striking a body, and being dragged. Scratches on HER wrists? How does that happen in a struggle. Seems more likely from dragging the body, either on the ground/rocks, or on pavement i.e. car.
I feel the probability of the above is low, but it is still possible. Would also explain the missing clothes, as they might be evidence of a car accident, i.e. road burn from being dragged.