I see what you're saying - but for the sake of this, lets say the tests come back positive for hallucinogen substances on the MJ, does that eliminate the possibility of him having a car crash and having a head injury and passing in the immediate area? I don't believe it does.
If the tests for hallucinogen substances in the MJ come back negative, does that eliminate the possibility of him ingesting something entirely different back in SM? I don't think so either.
Without the body, nothing can be confirmed from the items we are aware of. I am not even sure if after this long tox reports would come back conclusive (admittedly, i do not know the science behind this area of forensics).
I don't think we can even say for certain he "stripped down", whose to say he didn't change before he left and he threw those clothes in his backpack? We know nothing for 100% fact other than when his phone pinged and where (whether or not he was in possession of that phone can't even be guaranteed (based on what we know)), where the car was found, and the items within it. Other than that, I think ALL possibilities should remain open.