...This mostly what the last few chapters were leading up, but why did she end up in a third timeline?
Yeah, it's a rushed ending that leaves a fuckton of questions unanswered.
Like: if Seto doesn't know about Nanasawa's timeslip accident, it means she never met adult Nanasawa when she was a teen. But then, how did teen Seto and teen Nanasawa get together?
Or: if this timeline had a Nanasawa who had been together with Seto for 10 years, where did she go? Or rather, where did her memories of 10 years of being together with Seto go? Because this Nanasawa doesn't remember anything of that; she woke up in the morning in what she thinks is a strange house and has zero memories of the life she's been leading for the past 10 years.
And: is it really that simple for Seto to accept that her lover, the woman with whom she had been living since their high school graduation, has basically lost all the memories they made together for the past 10 years?