Were there signs that the dead out ever discovered the sugar? Sometimes they don't :(.
Was 'any' honey still available or was it pretty much gone? Did you find bees still in cluster or just dead on the bottom?
Were you able to locate the queen in the pile of bees? Did you have 'any' capped brood to investigate other possibilities besides starvation?
When you got them ready for winter, was there 'at least' one packed super of honey above the broodnest, or minimally 5 frames centered above broodiest in the box?
I noticed you said that you 'wintered' in two deeps, right? Did you make sure that the majority of honey left for them was 'above' broodnest and not the sides or (worse case) below?
Sorry if some of these Q are repetitive.
**Bees consume 'more' when they are active, even temps in the 30's will have bees consuming great amounts rather quickly, its why I'm usually more concerned when winter temps are above average as opposed to below average.
Cold doesn't kill bees, as has been said, moisture does. Fluctuating temps will cause a freeze and thaw inside any bee hive to some extent (drip, drip, drip). Its remains the 'thaw' for Northern beeks that should bring the most concern and action IMO. If you've got bees 'flying' in December or January in the North you are the only one that can make certain your bees have stores to last until the flows begin. Once feeding begins it mustn't stop until dandelions appear.