I just don't think it's an impossibility.
The driver said he'd left her alone, and that no one else was around, so I think that if this was a homicide, the best suspect is West because otherwise, it would have been a very opportunistic crime, and I think the circumstances lessens the probability that it was.
The sticking point for me, though, is that there would have been injuries to the body if this was a homicide... some sort of evidence of an assault, and according to the coroner, there wasn't any.
I think you should read a little more about the case ( particularly about the suspects, the events leading to Thelma's death ) before reaching a conclusion.
I respect your opinion, but remember that a coroner/autopsy report don't always tell the entire story.