I see, still there is no mention of him going to college, I assume he is going to college after high school during his time with Niki, from what I gathered it's a 4 years college course, which aligns with the 5 years with Niki.
If the theory that Ami is his daughter is correct he would have to be with Sekaiat least since his 16 so revised timeline would be like:
1)He is 15, hurts his leg, gets close to Sekai.
2)He is 16 Sekai and Nozomu marry, Ami is born, he meets Niki and starts tutoring her.
3)18 he starts going out with Niki, and begins his college course.
4)5 years go by, he is 23, Ami is 7, Nozomu and Sekai dies on accident, he and Ami disappear the first time.
5)3 years later IIRC Noriko finds him tutoring on the Old District and begins being tutored.
6)Maya begins being tutored, but somehow is kept secret from Ami.
7)Ami befriends Ayane in middle school.
8)Something happens he disappears again.
9)They go live in Nozomu and Sekai old apartment, and he starts teaching in Kumon-mi High.
Somethng like that.