RSBM
In this case, two obvious murders, I don't believe TOD would be based solely on AM's witness statement. I think they (not sure who 'they' would be, but a doctor, at any rate, not the coroner who is an elected lay person), would have collected medical data, particularly asap the temperature of the bodies, degree of rigor mortis, and other key physical markers, and sent this data with the bodies for autopsy.
I think, clearly, the bodies were very recently dead. Hence the potential time of 9:30. They clearly hadn't been dead very long. However, the 9 pm at the earliest cut off may have come from AM's statement.
At any rate, it doesn't prevent police from proposing an earlier time for the crime, if they disbelieve the witness. But not by much, they can't argue the bodies were there for hours.
JMO