You seem to imagine that illegal drugs are produced with as much care as legal drugs. If what you say were true we wouldn't see a huge spike in overdose deaths from fentanyl.
We're not dealing with a smart person here.
Here's another thought experiment:
You can either have someone kneel on your shoulder for a few minutes.
OR
You can be given a dose of fentanyl 3x over the lethal limit.
Fentanyl is NOT an illegal drug, it can be made in pharmaceutical labs perfectly legally but Floyd was taking it without a legal prescription by a Dr. The spike in deaths was due to the fact that they were selling it and saying it was heroin and people didn't know.
I've known many addicts with very high IQs. It's emotional damage that causes addiction. If the cop just had his knee on the guys shoulder there wouldn't be this court case.
There's no doubt Chauvin's defence will argue that we can't definitively know what killed him.A good lawyer will be able to put enough doubt in a jury's mind after all (and especially considering what he's facing) it's got to be beyond reasonable doubt.