Daniel was acting very strangely, both relative to how most people act and according to people close to him, strangely compared to his typical behavior. His final known act is very strange. Just taking off from work into the desert after talking nonsense to someone he just met on the job.
That he then disappears makes me feel like foul play would be a huge surprise given that he seemed headed down an odd path at this time. More expert evaluation of the crash data would be great, of course. But to me it seems the most likely thing is that he crashed the vehicle while driving aimlessly, then in an effort to get it unstuck from somewhere he turned it on several times and spun the wheels a lot, thereby putting mileage on the car without going anywhere. Of course he may have crashed and just drove around afterwards in a haze and/or looking for help. I wonder if the heat prompted him to remove his clothing.
I'm wary of the PI in a case like this since he has a financial interest in appearing to offer completely different information than law enforcement. Parents are rightfully frustrated with the inability to find out where their son is and may find it harder to accept that the son was in a bad state of mind and made some mistakes compared to the alternative scenario that a bad man took him away. Obviously if it was your child you would want to chase down every lead that might bring your child back safe and if not, leads that will bring justice to anyone who did them harm.