This is a theory of mine--it is continually brought up by poster after poster as if they're disturbed by the possibility there could be any other scenario than what they envision. Not sure why this keeps getting rehashed and rehashed, but since you've asked, a child buckled into the back seat of a car would also be in a sitting position when rigor set in. He would have been able to scratch himself and he would have been able to beat his head against the back of the seat. It is questionable whether a child would be able to work its way out of a seat belt, but I don't know how Cooper reacted to the excruciating heat--it's possible he could have become extremely fatigued, may have gone into seizures, may have become totally disoriented--all which would effect his ability to get free from the belt.
Why would RH lie about buckling Cooper into his car seat? Maybe something to do with child seat belt laws comes to mind....
I don't really feel a need to rehash this over and over, I made a post on a different thread where no one responded in such a way as they have on here. But if you want to keep talking about it we can...as I mentioned before and as you said, the forensics will close this one way or the other. I guess it's a disliked theory because if RH didn't buckle Cooper into that small car seat, but put him in the backseat, it may take some heat off the theories that he intentionally killed his son?