People should be put into the recovery position on their sides

As a former EMT I know this can't be done with a cuffed suspect. Given his altered state of consciousness, he was an immediate danger to himself and the bystanders around him. Therefore, he could not be released from the cuffs either. Ambulances have soft restraints that they can use to restrain a suspect to the stretcher, but little could be done until the ambulance arrived.

a knee to the back between shoulder blades

Pressure on the chest cavity can absolutely impede breathing in a weak or sedated individual. As long as there is no previous neck trauma, knee on neck, as Chauvin did, is not life threatening. It's super uncomfortable and can lead to non-life threatening neck trauma.

"Sudden In Custody Death Syndrome"

Most people have never been restrained in their adult life. Some people freak out to the point of unconsciousness just from being placed in cuffs. This even happens to people who were calm right up to the instant of being cuffed. If the person had drugs in them or heart problems then it can definitely be fatal. However, Floyd served time in prison, so he should have been able to handle being cuffed.