OK I think I can explain the recent Nikaido scenes. You see you don't just stop loving so fast sometimes. So I think author wants to show the development of their new dynamic now that the situation has been established.
Nikaido moments are so hard to watch. As many said we already have the 2 main character confess and kiss each other.
Why do this? Have her move on of something. This is not a harem Manga after all.