Ok here's a few pictures.
The photo of the bottom board has a lot of sugar on top
of the dead bees, so those bees were dead before the sugar fell down on top of them. The shots of brood all have all the classic look of PMS. Some of the frames are full of white specs, which look like sugar granules. Where there is brood, I see perforated and sunken cappings, and not a trace of honey or other food for the bees in those areas. the bottom board shot suggests the colony started out with a decent size population, lots of dead bees there.
There is no feed anywhere near where the brood bits are.
My guess as to what happened, adding all this up. Decent size cluster, compromised winter bees. The cluster was sitting over a brood patch, in contact with feed initially. When it got cold, bees started dieing off faster than typical, cluster shrank, lost contact with the feed , then one by one the compromised bees died off. Not a typical case of 'starve beside food' as I see no butts sticking out of the combs. IMG_6939 is a bit bothersome, to me that doesn't look like a good pollen frame, it looks like a frame full of pollen mummies which are no use to the bees. The frames with honey are also bothersome, there is no sign of the bees having moved the cluster over to those areas with feed.
This is a case that I describe to folks as 'The bees were unhealthy in the fall, colony already dead, just didn't know it yet', and after the winter arrived, they found out. It's very typical around our area that we have a warm spell in December or January, folks see lots of bees flying, lots of yellow spots in the snow. Then in late January of February we get a cold snap with a decent dump of snow, and when it warms up again, colonies look just like this one.