With such "revivals" they weren't fully dead yet. They would be, if nobody intervened.

Human bodies are irrecoverable once life fully ceases. Before that happens revival is still possible. Afterwards, it isn't.

There also have not been any revivals after brain activity stops. Once neurons stop firing (which happens on cell death), no revival is possible anymore. At that point, death is permanent.

That's literally not how it works.

Propofol slows down your brain and changes several "rhythms", but your brain activity does not cease at any point. To your conscious self it may appear as if you've "paused", but this is absolutely not the case.

Hamsters and small animals can be partially frozen and unfrozen. Once fully frozen (or close to it) they can no longer be revived. Once brain cells die, they cannot be revived.

His study showed that every animal in which 15% or less of the body water had been frozen recovered completely. Two-thirds of those in which 15 to 40%, and one-third of those in which 40 to 50% of the water had been frozen were fully resuscitated and survived long periods.

Hamsters in which 55 to 70% of the water had been frozen subsequently recovered heart beats and breathing but not consciousness, whereas when 76% of the body water had been in the form of ice resumption of heart beat was the only sign of life.