So you are saying the hanger can be removed from the bolt threads? but the bolt itself does not rotate inside the RD knuckle? From the pdf
if the hanger is removed there should be a U clip that retains the bolt in the RD knuckle. Pop the U clip off and the bolt assembly should be
removable from the knuckle. Shimano is not very forth coming with repair parts not regarded as things that wear out like cables, chains
however so your best bet will be a new RD. You asked earlier about using a 5700, and that would work fine. The RD doesn't really care
if the cassette/chain is 8-9-10 spd.

From another POV, you might consider finding a junker bike for winter use, one you can comfortably discard after 1-2 seasons. This
approach has been suggested by a number of Canadians who ride all winter on salted roads. The other option is fanatical maintenance,
washing the salt off and regular relubing down the the constituent parts, every threaded junction and ball bearing.