(Screw the morality of two-timing with the same person, this is a lot more ominous)
Sasaki's dreams of the future is becoming more realistic. Seeing the injury on her thumb propagating into the waking world, it might be that she's slowly transitioning back into her original timeline. With that prospect, she might... not have that much time left.
Dammit... Old Nana-chan, settle all of your concerns in that time period. It's true bliss, being together with Saki and young Nana-chan and everyone, but if it's going to be over soon then you've got to end it with no loose ends and bitter regrets.
This "finality" vibe is making me tear up. I hope everything's going to be okay. I'll put my faith in you, you horny wholesome Nanasawa dork. (*cries)