I believe you can prune them back really anytime over the winter. I did mine on a warmish day in February.

I also pruned mine back in late February, some of them were already very tall, as we had a very mild winter this year. Of course we have had a freeze since then but it hasn't seemed to effect them. I usually treat all of mine as group 3, as they usually freeze to the ground. The years I have lightly pruned my group 2 they have later froze and looked very ugly and spindly.

Not all. You can safely prune back clematis at any time but in winter when they are still mostly dormant is often the preferred time.

I prune mine any time between when we have a hard freeze that stays and when I start seeing the buds just begin to swell in the spring. I prefer doing it sooner if I can so I don't have to look at dead vines all winter.