I hear you -- this is where I wish we had a medical examiner on WS!
OK, brace yourself, I'm about to admit my humanity and that my thinking is evolving here. I agree that there are a number of things you're not going to find unless you're looking for them. And I admit that my own "suspension of disbelief" is based largely on stats and the fact that I don't think our primary actors in this case are smart enough or lucky enough to have killed a trucker in a way that would seem totally compatible with his undiagnosed medical condition that they didn't know about.
BUT, that said, I have read stuff nonstop all afternoon, bouncing back and forth between work (ha), this forum, and a bunch of NIH research. Most recently some stuff on autopsies and DVT. I am starting to think it will really boil down to a detail tox screen, no matter what the DVT evidence is. Because guess what? The number of undiagnosed DVT cases who die below age 60 is startlingly high. What I'm saying is that if they did a second autopsy on AxC, I think it likely they would fine evidence of DVT, and it STILL wouldn't mean he wasn't poisoned.
It will be morbidly ironic if he WAS poisoned, and the perpetrator just managed to get lucky enough to kill him with the same thing he might have died of a little later on anyway.