Taking Charles example, there is an unequivocal link between him and his long deceased grand-father that he never met, so it is logical to call him grand-father, the same way he would call any ancestor he would have found from the 12th century.
But there is no link if, f.i., he would have married a woman with no living father or mother he could call father or mother-in-law.
It would be more a sort of social use of the expression.
It certainly is easier to say "my father-in-law" instead of "my wife's deceased father".
I've heard that if you divorce or remarry after being widower, your wife's parents (and siblings) remain your in-laws, meaning you can have 2,3,4, etc mothers-in-law
Anyway, in-laws are not blood relatives (usually), as Jane well put it, and there aren't any legal bonds towards them, just social ones, so it is a flexible category IMHO
I don't know ... it's confusing, because in another article, Wikipedia referred to Princess Victoria of Saxe-Coburg and Gotha (Kohary) as the *mother-in-law* of Duchess Sophie Charlotte in Bavaria, despite the fact that she died young (in 1857, aged 35) -- before any of her children married. So I see a complication, here, since Winifred Wagner is referred to simply as the wife of Siegfried -- not daughter-in-law of Richard (although she most certainly was Cosima's daughter-in-law).
Another complication is if the spouse dies, especially if the person remarries. Interestingly enough, the composer Giuseppe Verdi continued referring to Antonio Barezzi (whose daughter Margherita he had been briefly married to) as his *father-in-law*, even after his wife's death. He remarried many years later, with Giuseppe Strepponi, when Barezzi was still living. One presumes that he no longer referred to the older man thus ...
In fact, I've heard of the expression "former mother-in-law", applicable to persons who have been widowed or divorced.