I agree with everything you are saying and more. This whole scenario of her falling out of the car and his "assumed" thought of Janice travelling in the opposite direction in another vehicle is based on his sworn testimony.
I am under the impression that by him taking her for a drive, then attempting to have sex with her, would seem more likely the reason why an argument took place. She was possibly rejecting him, and her only defense (again, I am just trying to use common sense) was to kick , scratch and punch him, to keep him away. I think it is very important to gather a map of the area, along with a timeline. I am wondering if the same route from the motel, to where he said she fell out, is in the same path he took going to and from the park.
I suppose there could be a possibility that Janice may have opened the door to try and escape his intention, and may have also hitchhiked back to the motel, but she was never seen again.
If we go by the sworn testimony of Llloyd Rach, then you are right, the whole thing does seem fishy to me as well.