IMO, it'll all end up being a dream she had after fainting.
But then, she'll confess in present time.
Although the dream ending is a classic, I believe there are reasons why it is unlikely to be used here.
1. The story made a big deal about how much Seto suffered and that Nanasawa the older's motivation was to prevent that bad future. The writing until now does not inspire confidence in the idea that it will have a bittersweet compromise.
2. This can be a stylistic story telling choice, but the fact that we followed different perspectives several times makes it harder to accept that it was all Nanasawa's dream, especially because she doesn't know the scenes that happen without her around. I know that logic is not required in fiction, but it woud feel dissonant nontheless.
That being said, I cannot believe the future will not play a part at all after this point, so my guess is that "Sasaki"'s mind will be sent to the future of the new timeline Nanasawa and she will be together with Seto. Whether she will remember the new events from the timeline and completely meld with her new life or if she will remember only what she herself experienced is a different matter.
This is a barely coherent time travel story with an adult constantly sexually harrassing and assaulting a high school girl
Nanasawa both is an adult and a high school girl (obviously—we see them right there on the page), but Seto is also both a high-school student and an adult.
One might argue that Seto only will be an adult, but that’s the oxymoron: an entity cannot be in the state that it is and the state that it will be simultaneously—except in fiction.
Seto is not a time traveler, physically and mentally she is a high school girl. Future Seto is irrelevant to the matter. And just because Sasaki is molesting her crush while thinking of her adult version we can't delude ourselves the same way (I hope),
I understand that you are making a joke based on the mind-boggling conveniences of time travel, but even within this fictional logic that does simply not apply.
110%, and very soundly put but.. but if you can just iron out the creases in your brain enough, you can see the funny side if not in the comedy, the comical nature of just how badly hashed together the MC is.