Don't apologize! Your post was perfectly readable!
I think everyone's forgotten that all of this started by Miyuki complaining to her friends that her relationship with Eiji hadn't "progressed" very much (which is a highly normal complaint for someone in their first relationship) and said best friend (I don't know her name) misunderstood her complaints and introduced her to Kondo for relationship advice. Kondo, who already has stated that he likes ruining people's lives (chapter 2), started to easily manipulate her because he was disgusted by Eiji and Miyuki's "pure" relationship.
My stance has always been that I'm not absolving Miyuki of her wrongdoing. By no means at all. She's a greedy coward. She wants to have all the fun of having a "mature" relationship (read - have sex) with Kondo while Eiji is on standby to marry when she's had her fun, and she wants to maintain her image and reputation.
HOWEVER, that being said, it's very clear from chapters 1 - 3 that Kondo has habitually done this to other girls, which means that the vast majority of the blame lies with him.
Heck, we can go back to chapter 1, pages 38 - 41 actively shows Kondo giving Miyuki the progression she wanted from Eiji. Yes, we can look at this whole situation through the framework that Miyuki should have known better, but, again, that's where we have to look at Miyuki's internal monologue "Unlike Eiji and Me, who were each other's first loves, [Kondo-senpai] had a much more mature view of relationships...I think I was the perfect mark..." (Chapter 1, pages 38 - 40). We can look at this internal monologue as future Miyuki looking back and realizing that she was manipulated.
Which she was.
All of that to say that emotionally abused people (which Miyuki is), will almost always go back to their abuser. It's very plain to see when Kondo puts her on the spot to choose between him or Eiji, and the abuse started when Kondo casually implied that he wouldn't see Miyuki anymore after they had sex.
Chapter 1, page 41
Kondo: "...Hey, the 30th - the club's off. Let's hang out."
Miyuki: "Ah...um...That day is...His birthday."
Kondo: [cold, deadpan expression] "Hmph. Whatever. I'll ask another girl" (Abusive Statement.)
Miyuki: "Huh...?"
Kondo: "I mean, come on. You're always putting that Eiji guy first...It made me realize that I barely existed in your world. I'm tired of chasing you...Let's just end this. This way, we can both be happier". (Abusive Statement. Translation - "Since you're not going to do what I want, I'm going to end this unless you comply")
Miyuki: "W-what are you saying? W-wait a second!"
This scene is where Miyuki folds because she's been emotionally manipulated for weeks by Kondo, and she's had sex with him multiple times. She was put into a vulnerable position, and because she's tragically in her first relationship, she doesn't know how to look out for the signs that she's being manipulated.
Then, like I said in a previous post, she then starts to double-, triple-, and quadruple- down on trying to keep both her reputation and Eiji.
I will push back on your metaphor of a cheeseburger. Kondo's manipulation of Miyuki is more like a puppet master. Kondo tied strings around Miyuki's arms, legs, and neck, so she would dance to
his tune, and when she pushed back, he threatened to cut the strings, and because Miyuki's been so conditioned by Kondo, she willingly dances to the way he pulls her strings, even though she actively realizes that she's in the wrong.