Ya but it's just stupid enough and convoluted enough to work. The MC is pretty dense and she has literally been throwing herself at him I think she is just frustrated. But what ever the big mystery is it's not going to be satisfying or a worthwhile excuse to her actions
OK so my theory is this for sake of being as tragic as possible ending:
Umimi - ends up alone "without family with MC" she wished for, possibly cause she did something stupid (even worse than her SA/Rape attempts) and was shunned or even jailed.
Shizuku and Takt are in their arranged loveless marriage - loveless cause Takt loves her, but she doesn't love him, and is forever "angry" that she didn't win MC.
MC ends up with Yako - which is a sad ending - cause Yako the only sane girl "won", but what she won is being a couple with a dense mofo who will never stop loving Shizuku, so she is sad in this relationship, with potential breakup to put a cherry on top of the sadness.
If we want to increase the tragic-ness one or more of the characters could "play sudoku".
This would explain why Takt tried dating Umimi - to forget Shizuku - but it didn't work. Later he either won her, when she gave up trying to make MC fall for her, or Takt used his parents to get the arranged marriage thing and get Shizuku by force. Maybe even gave her the "door" to run away, if she managed to lure MC, but MCs friendship with asshole Takt won every time. Umimi/Yako didn't manage to overwrite MCs love for Shizuku.
MC find out why Shizu/Takt are married, how Takt never was his real friend, how he lost his crush to his "idealized" friend, cause he would rather protect male friendship with a fraud rather than "damage it" and go after his love/crush. Shizu sad cause forced marriage, Takt sad cause he won, but the girl hates him so the sex/stuff is just miserable.