I'm 95% sure that a Masuzu end is locked in now.
Close to the end of the story, the standard drama arc postulates that the heroine and/or the hero have to be on their knees, and that's where they are now. Both of them, in different ways.
Masuzu is finally realizing her own feelings, and so are Hime and Eita. Keep in mind: Hime is not implicitly ceding Eita to Chiwa, but to Masuzu. Eita is finally adding up 2+2 with Hime's help and determined to follow up, staying at Masuzu's side in whatever form (lover, fake boyfriend, accomplice). It's only Masuzu's stubborn facade which is remaining, and it's beginning to crumble.
Back on her own, Masuzu's reminiscence over Eita is turning from petulant rejection to grudging acceptance, and in the end, genuine tears. In my interpretation, she realizes that "love" can be a beautiful, touching thing, and not the emotional disease she has been treating it up to now. She had already admitted in the past that she's feeling envy for the "genuine" girls - her anti-romance stance is limiting her and forcing her into isolation. But she can't deny her pseudo-conviction and come clean.
Her final misconception is Eita's relationship with Chiwa, where Eita's genuine care for her moved her to tears. Her willingness to let go of him in the end is rooted in her decision not to "stand in their way" anymore after watching the kiss, and in a way, it's one of the few genuinely selfless gestures she ever did. However, it's just a final outburst of self-delusion, because in the end, Eita thinks of Masuzu entirely differently compared to his thoughts of Chiwa.
Chiwa is family, someone he deeply cares for, like family. She's been less of an osananajimi, but more of a little sister to him. However, I'm not aware of any incident where he had romantic feelings or sexual tensions towards her. Very different to the various scenes with Masuzu.
I'm pretty sure that the next volume will be the final or next-to-final one and be very Masuzu/Eita-centric. She's the girl whose wounds have yet to be healed, and whose growing feelings to Eita have increasingly undermined her own conviction. This is why she's become weaker and weaker, and more susceptible to counterfire.
So, she'll have to have to suffer till she breaks and acknowledges the truth that she DOES love Eita. Wonderful dramatic stuff. I'm sure that Eita has the tenacity which is required to break through to Masuzu.
Looking forward to the conclusion
