The fact that the bodies are so intact should enable forensic pathologists to determine from tissue samples whether the passengers died from asphyxiation high in the air, or whether they perished as the plane sank far beneath the ocean. This, in turn, could provide clues as to what happened
No one died from asphyxiation high in the air. They died from the impact at the surface of the ocean or from drowning as the wreckage sank. There is no evidence of depressurization from the recovered wreckage or in the ACARS record until late in the sequence and by then the aircraft was just about in the water.
My vote is that the impact was sufficient to impart fatal injuries to all. A conscious survivor would not be strapped in a seat for long.