This is insane, seeing that divorce petition really shocked me too. It's insane she chose to divorce her husband and throw it all away just like that. It also hurts because Harutaka was a good guy and I can't help but wonder why she chose Hana over him, but it's fine I guess. It's also insane because throughout the story you don't really get to see or learn of Hikaru's sexuality, it's not so evident, so she straight up going for a divorce means explicitly she knew her feelings for Hana were romantic, even if that wasn't exactly shown or developed either.
While I think it's rushed, I like how decided the characters in this manga are and how keen the author is on driving the story where they want even if it means hurting characters that haven't done anything wrong (that's life I guess) and it still doesn't feel unrealistic