.....Is this an accident or an emergency in your opinion ?
If there was the possibility that the victim was still alive/ could be saved, I would for sure say it was an emergency.
Speaking of accident.. it is harder to tell. We would have to know the reason for him being unresponsive and having lost the regulator.
But as I mentioned before, I think that for our every-day-use it is not that important to be exact about the definition. That´s rather something for the police, lawyers, judges etc.
Quite often it is a mix of "events" anyway. A further non-diving- example.
A person with known problems with his heart suffers an heart-attack while driving, dies, hits another car the other driver dies as well.
Would it be an emergency? YES, the main reason being that others are also endangered as long as the situation persists and usually bystanders will not be likely to identify whether a person could be saved or is beyond it.
Would it be an accident? Definitely YES. BUT: Driver 1 would not be victim of an accident (but cause). Driver 2 would be a victim of an accident.
Press/ Media would report: Accident with 2 casualties.
Bystanders would most likely say the same and it is totally ok.