I think most of the suggestions I've heard from broadcasters has been 'first husband', so I suspect, when the situation eventually arises, that will be what is used.
I think it has been common for awhile to refer to the president as 'Mr <surname>' in writing, typically after an initial 'President <surname>'. I suspect the various news outlets have their style manuals which dictate how to use it. Our language has an aversion to repetition so 'Mr <surname>' provides an alternative to 'President <surname" and 'the president'. Incidentally, when I was in school, when referring to the president of the U.S., it was always supposed to be capitalised, but back in the 70s or 80s, most style manuals changed it to only being capitalised when used with the surname.