I wold think the pressure on the chest would exert pressure on the heart..like squeezing a tube of tooth paste.
But wouldn't the pressure have been distributed in the whole body, then ? The arms and legs acting swelling at the same time as the head tissues, acting as reservoirs ?
Maybe the cold change the logic : with shrunk blood vessels, less blood can got there. More goes to the head. But still : i'm not convinced at all. Would they have seen abundant nose bleeding, ear bleeding, after all this time in the water ? I can't imagine the eyes giving up before the nose vessels...
As far as a snow event.. A) they would have known 10 minutes in to the walk to the woods that there was no snow event and gone back, or, B) they would have known that it was over and gone back
A) If I was fearing an avalanche to the point to move in the middle of the night, nop, 10 minutes would not change my mind. Especially in those conditions, where it's not only about the snow on the slop it-self, but about the drift/overhang (it's not step enough to have an overhang I think, but a drift can do the same job) : as long a there's wind, loading the drift at the summit, the danger is increasing, not decreasing.
B) On avalanche doesn't mean there won't be any other. On the contrary, if one slab is sliding, the other around can be weakened...
If the tent had been quite under the avalanche, it would have safe to go back (but then good luck to find it). If only one slab on the slide had been moving... well, I'm not voluunter ! ^^