In all those sentences you need the plural. The antecedent of "who" is "men" and "popes."
The clearest example is (3): Pope Francis cannot have led the Catholic Church for almost 2000 years! (he's only 81 years old).
The structure you present us with will only allow for the singular when the postmodifier (i.e. the "of"-phrase containing the plural noun) refers to a restricted group. If the group is large, the relative clause will most likely be used to define it: men who are always courteous (i.e. courteous men). If the group is limited in number, the relative will refer to the singular subject or to the group depending on what precedes the "of"-phrase:
- John is the only one of my friends who likes classical music. (John likes classical music, but my other friends don't.)
- John is one of those friends of mine who like classical music. (Jonh is one of those classical music lovers.)