As a big proponent of the theory I have to admit I mostly just throw it out for the joke at this point. I could see it possibly happening but maybe not. I'm pretty sure at some point MC gets stuck in the past and has to live back until the current date which is where the old man comes from. At this point the old man is either him that has lived through the 20 years or his dad and I'm doubting its his dad. Revealing that the girl is Darcy only reinforces my belief that the future man is an older MC. I'm thinking he's having some sort of time loop issue where he keeps trying to fix the past but keeps fucking it up. And during all that time he fucks and fathers all the girls. End of theory.
I don't think he's Jessica's dad though? I think Tiffany would recognize him unless at some point you travel further back in time but I feel like the time portals are a fixed 20 years apart because time keeps progressing the same on both sides. It'd be double incest if Jessica was since he'd be fathering her with his own mom.