I don't think rigor mortis would have gone away that soon if it had been able to set in.
I'm really sorry for your loss though, and thats bizarre about the rat losing hair. While they may not be able to do a full necropsy, if there is some serious viral infection or something of the type, would they not be able to test for it with the remains that are left?
While you may not be able to do anything if your other snakes are also affected, you may still want to at least speak with a vet and see what they may recommend.