He was fired because he didn't follow training.I'm not so sure.
I think the physical evidence will likely support the heart attack and drug OD argument.
It seems that there may not be any physical evidence to support any other cause.
I personally don't think we can say that Chauvin's action contributed to the death - the medical experts and the jury will have to figure that out.
I suspect that if the cops had gotten there 15-20 minutes later, they wouldn't have been arresting him, they would have been trying to revive him.
Here's why they restrained him that way as I gather from some of the recently released transcripts:
When they had him in the vehicle, he was flailing around and banging his head and face against the interior of the vehicle.
He had lacerated his mouth and was causing himself harm.
They removed him from the vehicle and restrained him in such a way that he could not continue injuring himself - most particularly beating his head against the pavement.
They had called an ambulance early on in the interaction, they were trying to keep him immobilized until it arrived.
Had they not done that, he would have been slamming his head and face against the pavement, and that would be the thing we're blaming them for today as contributing to his death - inaction rather than action.
I suspect that the verdict will depend a lot on assessments of how closely Chauvin followed the training and accepted procedure for using that restraint.
My personal feeling as an uneducated observer is that they acted properly and within the scope of their training; maybe the experts in the matter will contradict and disprove that.