Who is (or are) the main character(s) in this story and why do you think so?
I would say the older waiter is the main character. It is he who has that burst of existential angst near the end of the story. Readers see into his heart. Everyone else is just a foil to produce this moment.
I guess the old waiter is the main character of the story. It's him who has been present in the story from its begginning to its end. and while he's been talking about the old man it seems that he's talking about him self, even the writer is doing so, we can see that he's talking about the old waiter's personnality through the old man.