Wait, wait, wait....... I almost missed out andddddd Mashuu grew up healthy and handsome!
I have two hunches that are difficult to side with.
I have heard the author assure that there is no special relationship between Kotoko and Mashuu, and Mashuu might end up with his classmates. But I think this is also unfair because the author always gives strong signs of psychological closeness. It's clear there is something between them. Kotoko is not stupid enough to always be cheated to feel comfortable around him, even more than that, even if a little. They are floating not making any progress. They just don't make it work.
I even don't think Mashuu is playing around about his feelings. He is honest with his complexity to monopolize like a little kid do: this is what makes me wonder if this will shift the potential end of the story. If in the end their strong bond was only a realization for Mashuu and Kotoko only became a desirable parent figure for him, then the little girl was clearly his true partner.