After reading the account/police report I tend to agree with your analysis. She probably is/was a psycho that kept pushing the guy until one time he snapped and hit back. She probably fell and hit her head. Now, she is buried out in the desert somewhere.
OR, he kicked her out of the van and she got picked up by a predator. Then, that guy killed her and buried her in the desert.
The fact is IF she was alive, she would have called mom and daddy to come and get her.
It may not have escalated to violence. He may have just left her somewhere. Or she may have stormed out of the relationship. Perhaps into the desert in the middle of the night. He might have reached his limit and drove off, calling her bluff.
She may yet turn up in the company of new companions. Or her bones may be bleaching in the sun where she expired on her own. We may or may not ever know.