The police are testifying voluntarily or they are under a subpoena and not doing so would make them in contempt of court. “The mob” has nothing to do with it.
It’s a given Chauvin killed Floyd because the footage (multiple camera angles, both from body cam and bystanders) is not in dispute and the timeline is obvious.
9.5 minutes on the neck: next moment, on a stretcher, in an ambulance, and dead.
It’s possible Floyd would have survived the encounter were he not on drugs that have a tendency to suppress the respiratory system. Or he may have died anyway.
It’s also possible Floyd would have survived the encounter if he were white, or a woman, or both, because then he likely would not have been rough-handled in this way.
All we know is the facts and the facts point inexorably toward Chauvin’s guilt at some level. Murder 2nd or manslaughter.