Did you monitor the mite levels in the summer and into the fall? Of course monitoring, alone, doesn't save bees but knowing the levels gives you a good starting point in sorting out what really happened to them.
Although mites are often the culprits, I also think that some times they get blamed when there's other significant contributing reasons. While different decisions re treating might make a difference in the future, not identifying ALL the factors can continue to leave you vulnerable when you unknowingly repeat the same things. But really knowing your mite levels (by using a regular monitoring program) gives you a way to judge if they are likely to be the main cause, or not.
So may I suggest two things to consider for the next season: 1) do regular mite monitoring. I happen to find sticky boards useful and extremely easy to use. I do 72-hour checks at least once per week, and that gives me a very good idea of what's happening with the mites. It doesn't force any action (so it's compatible with TF beekeeping, as well as for a beeekeeper who treats. If you need some tips on how to integrate it into your routine, I'd be glad to describe what I do.
The second thing I have found useful is having shavings-filled quilt boxes on top of each of my hives. These are easy, inexpensive tools for managing moisture within the hive during winter, and are surprisingly heat retentive. I had assumed that they would just function as moisture-regulating tools, but they also seem to have provided my bees with a cozy place to go. I, too, have sugar bricks in my hive (just below the quilt boxes) and on every day that isn't totally frigid my bees moved up into the space around the bricks to hang out (literally in festoons from the bottom of the quilt box), munch sugar bricks and stretch their legs.
I am north of you, somewhat north of Albany, NY and our winter has been awful, too. Cold, but more importantly I think, an extra 5-7 weeks of deep winter cold since it started so eerly and seems determined to hang on as long as possible. I've been promising my girls that the end of winter is near, but then we go back to frigid, again. I had minus 10 again last night.
I hope you will not interpret my suggestions (which are aimed at next summer/winter, not the past) as things you ought to have done, as I know you must feel pretty bummed by losing your bees. It seems to me that you did what you knew to do (leaving honey, adding sugar bricks, insulating top, etc.) And your bees did better than mine, which scarely made 2 deeps apiece, and one had only a single deep, so it seems your bees prospered under your care. But I'm making my suggestions in the hope they might be of some use to you in thinking through what you're going to do with your new bees (and all that lovely drawn comb - your new bees will be very lucky bugs!)
Enj.