Honestly I pretty much guarantee the ending will be something along the lines of "Older Nanasawa decides to let go of Seto for the sake of her younger self, and is subsequently transported back to her own time where she is now in a relationship with Seto in the present.
I had a similar thought
The main thing is Sasaki/Older Nanasawa will return to her time at the moment she left and decide to confess to Older Seto, with some helpful words from Young Seto to get the message through to the Older Seto.
Then some humorous reactions later from Older Seto learning about what her new girlfriend had done exactly heh.
And being jealous.