This series is really good, but, it's so complicated. Kind of reminds me of Usagi drop.
Well, I see at least 5 possible scenarios:
1. Nijima waits for Marika to be an adult and remarries her as he originally intended.
2. Both Marika and Nijima move on and Moriya manages to win Nijima over.
3. This date goes better than intended and Chika develops feelings for Nijima.
4. Marika eventually forgets about her previous life and Nijima never moves on.
5. Nijima moves on, but something happens to him before he develops a new relationship.
If Murata goes with option 1, it'd be Usagi Drop all over again, but, that'd really make the entire story pointless; I mean, Marika quite literally came back because she was worried about Nijima and if he doesn't change at all, then Nijima didn't grow since chapter 1.
As for the other scenarios, they're just some possibilities, Murata could very well go a route no one excepts.