It feels like this sort of explains why she's so antagonistic towards Eri- she basically repeats the same patterns as Senpai's mom.
It's sweet to get a reminder of how much she values Fuyuki, since the story mostly shows Fuyuki leaning on her.
There's a lot going on here, so I'm just going to point out that Fuyuki has said exactly what you said she should, and has never pulled the "it's ok if they abuse you because you have me" thing.
I think people just like to imagine a world where they get over their respective loves, because even as fake girlfriends they have a cute & supportive friendship, and good chemistry. It also helps that Senpai is hot.
Sister wanted to trick Marin into being a baby so she could raise her all over again, but didn't realize that she'd be traumatized out of drinking apple juice? :unsure: