In one of the Ruthless endings, the old man is in fact the MC, passing the deed on to a younger person, in almost direct mirror of the usual opening scene. I'm not sure if this is to be considered canon however, but it's worth mentioning.
Jean-Marie was stated to have brothers, so multiple, though the age of them is quite vague.
The old man being one of the gods is something that came across my mind several times throughout the story. You raise a good argument with the Zeus theory though, definitely adding that to my Minotaur Hotel thesis. :p
So, order of Master being handed down is:
Selected heir
Oldest male offspring
Oldest male sibling
Oldest living nephew
Oldest ascending male
Overseer, who picks a new Master
The Olympians had to vote to pick an Overseer, and in addition, only that Overseer may enter the hotel, while all gods are banned from entering it. This leads me to believe that, Hermes, himself, is the old man, simply disguised. I believe he was pretending to be Clement (just like how his human name is groanworthy, Jean-Marie Clement).
So, my theory is that Hermes is the Overseer, selected by the gods of Olympus. I don't believe the old man is anybody else.