Responses
Probably in the heat of argument Yuki got pushed down the stair and hit his head.
Yeah. I think so too. I also think that "Yuuta" seeing Yuki directly in the eyes at the end as a vision, means that he is close to remember who he is.
It's kind of weird. He has been telling him that they can't be together if he remembers him. Maybe Yuki was about to get married or something and he got really desperate and did something to him. I don't think it was just an accident. Maybe Tomohisa lost it in a moment, did something to Yuki like hitting him strong enough so he lost his memory and take him away. And now, the guilt is too much and wants to take back Yuki to his home.