The game doesn't explicitly say that the MC knows but there's literally zero chance that he doesn't, it's so obvious and the MC isn't an idiot. The dialogue and choices leading to it make it so clear that even a Japanese NTR protagonist would figure it out.
Not really likely in the future, even with every single sharing choice picked if you show him Ava and then take her away he makes it quite clear that MC's completely stolen his family from him. He's got a small dick, isn't good at sex, is poor, has no power and is completely unlikeable, he wouldn't be able to steal. All he gets are scraps. The last time Laura sees the old cuck is when the son, Cuck Jr comes into town and is on the phone with him. It comes off as if the dev just gave up on that entire storyline. I hate NTR and sharing so I never clicked on any of those choices until after your comment just to check it out and it just plain doesn't go anywhere really. What really comes from clicking the sharing scenes are scenes that really just reinforce the netori. Grandpa gets a taste of Laura and has her completely stolen away from him. Grandson is a premature ejaculator who can barely get it up and ends up forced to eat a creampie out of his wife's pussy.