Well, so one of his dead legs/feet broke off yesterday, so my brave spouse cut off the other dead leg/foot so that he'd be balanced sooner. There's about 1" of bone below the hock joint. We'll take care of him until spring, and see if there's any chance at a quality life. That doesn't seem super likely. I feel awful thinking that we're putting him through all this and he doesn't really know that it's not us harming him on purpose, but the results of frostbite. Euthanasia will probably be our course of action in time, but I'd like him to feel the sun before his life is over.
Thanks to everyone for the advice and sympathy over this ordeal.
Now we're just waiting and watching to see if the other 3 ferals survive the rest of our bitter MN winter. Warm thoughts and prayers for their well-being are welcome.