Fuuko's a hero in my heart, here to save Yuni from a doomed relationship with a neglectful Nanase. And how will she do that saving? I imagine we're going to see Nanase's friendship with Yuki, and Yuki's open affection for Nanase, exploited. After all, it gives Fuuko some amazing bait to reel in on the doubt that's going to always exist in Yuni's heart.
Things are already messy, but I'm looking forward to the coming storm. Fuuko's not going to let this lie by any means, I can guarantee it, it just wouldn't be in her character to let this stop her.
It's true that Fuuko is definitely "mentally sick," but it also makes her very interesting. Without her manipulation, any semblance of Nanase changing even remotely for the better never would've occurred. Yuni would've been trapped in a relationship where she would always feel like she was second place to her precious club idiot's sports club. Nanase should honestly thank her for getting Yuni to open up about how she felt about being neglected.
But what's most interesting about this chapter is doubtlessly that planetarium's strength. I expected it to shatter.