Is it every appropriate or justified for an officer to kneel on a suspect's neck?

Yes, but to hold a suspect down like that for more than a minute obviously has its risk factor.

Floyd had been subdued and cuffed. He should have been placed in a sitting position.

No, they just do it because it gives them a feeling of power and sublimated sexual gratification

Based on the comments about police training made by witnesses at the Chauvin Trial

This would be considered appropriate in the case of a suspect fighting with Police  but should end when the suspect is no longer fighting

which by the way may have been 

LONG AFTER

Police Chief Ramsey and many others believe the fighting ended!

We will see, when the defense arguments begin

No, such torture is not justified.  Especially when there are so many humane methods to restrain a violent criminal.  

That would depend on the training invested in the officer by the police dept. If that was standard prorocol, then yes. Fact is George should hav gotten into the car when he was lawfully arrested instead of resisting to the point 4 cops had to put him on the ground. His death is at least 50% his own fault. IMHO.

   However, I do think that cop did it far longer than he needed to.

Yes.  It is done to control a suspect.

I have no problem with it.  I've seen no validated study (or any study at all) showing it is a harmful position for the suspect.

I mean if the suspect is acting violently or irrational than yes.

Of course it's appropriate. Especially when the person is struggling to break free of an arrest. They could get up, pull a gun and kill the officer, or worse, an innocent bystander. However, kneeling on someone's neck after they've gone limp and unconscious is not appropriate.

I was told that it was not considered medical treatment of a person who is suffering from a drug overdose. No. And I suspect the officer knew when the patient passed out and went limp while the patient was under his knee. If placing a knee on a suspects neck is preventing the suspect from lashing out against the officer or maybe even himself, then I could see the need to hold them down, but not while handcuffed and unconscious. 

Obviously there is or they wouldn't train officers to do this.

The person isn't supposed to die as a result.

Thus the trial where both sides get to present their evidence to the court.