They cannot be mistaken about their Identity.
Even within the framework of JW belief, they have already been mistaken once. They taught clearly and unambiguously that the F&DS parable had been singularly fulfilled by C.T. Russell and they taught this for nearly a decade AFTER their own supposed appointing took place.
You can't argue with this simple fact of JW history. It is what it is. Of course they can be mistaken. They already have been.