Tom: Try one of those larger pyramid tarps and you'll have the comfort of dumping a lot of weight. They are perfectly safe and quite spacious and weatherproof.

IMO they are at their functional best in snow, given ability to easily bank edges, lack of insect concerns, and the potential for digging out floor for more headroom (which I've never done). Nothing like a lack of floor on deep subfreezing snow, when it's time to empty the pee bottle.

Lots of winter outdoor recreation programs use them. Canadian Alpine Club comes to mind and there are others.

They definitely aren't as warm as a completely double-walled tent, but the difference isn't huge.

No I've never camped above treeline in the winter and am unlikely to have a winter objective that would make this desirable. I've camped around treeline in winter less than a dozen times, all in Cascades.

The majority of my winter camping has been in areas of New York and New England which don't support a treeline.