Hi Elizabeth,
I'm certainly no expert on bees as I've never kept them, though know several permaculture folks here that do. My favourite bee book is 'The Buzz about Bees' by Jurgen Tautz,
http://www.amazon.co.uk/Buzz-about-Bees-Biology-Superorganism/dp/3642097502 in case you haven't come across that one. Beautiful and fascinating with lots of amazing close up pictures!
The weather here has been very difficult for bees this year, with a very wet spring in Britain. Many of our apple trees have no crop as the bees and other insects weren't able to fly in the heavy rainfall.
Bees I believe like to seek out hollow logs, which we tend to find in pretty stable sheltered conditions. Warmth is important, but the temperatures you've been seeing over there sound really harsh.
One possibly helpful perspective (I always like to seek these out) is that your surviving colonies will be especially hardy to such conditions, which may in the longer term be an important thing.
Have you tried different hives like the Warre?