Generally, you wait until Spring to prune down to live wood. If they were too weakened by BS to live through the winter you might look for more resistant varieties next time. Usually fertilize in the Spring. Good luck!

If there is new growth at the top of the cane it is still alive at the bottom. Even if it doesn't look like it. If the cane were dead it would not push out new growth above the dead section. Just leave them alone. In the spring you'll see that they will start to leaf back out at the bottom from any live cane that is there.

I wouldn't fertilize them any more now. You don't want them to push out a lot of new growth this late in the season. That growth for sure, will end up dying over the winter. You want the rose to store up and save all its energy to be able to send out new growth in the spring.