To start off, it seems that Naoi never felt any guilt over Kudou. She only agreed to the deal solely for the money and had always planned to cut Kudou off at the end of summer. A summer job basically. It seems all Kudou's pressure did was have Naoi settle on the quickest path to money. Obviously, Kudou has no idea that's the case. I'm sure she won't be happy when she finds out. Though with how things went this chapter, it seems like Naoi might've already cut Kudou off for good after the stunt with her phone. Speaking of, due to Kudou's actions causing Kurumi to be a received call and not a missed one, she caused Kurumi to be trapped by Kokoro for hours. I'm guessing Kudou either thought the call wasn't a big deal, or didn't want to risk Naoi leaving before her humiliation session could start. Either way, it feels like Kuwabara is trying really hard to turn Kudou into a hatesink.
Naoi's super angry at Kudou, but also likely getting worried as Kurumi obviously won't be able to pick up any calls from her, or even realize she was trying to contact her in the first place due to Kokoro. It's hard to say what Naoi might walk into. However, Kuwabara had Kokoro stay passive long enough for Naoi to make it to Kurumi's house before anything really happens. Notice that Kokoro and Kurumi weren't even allowed to have their first date before the story split them up, since Kuwabara wanted Kurumi's first date to be with Naoi. I don't think Kokoro's getting any first times this time either.
As for Kurumi, she was basically held prisoner by Kokoro in her own house from breakfast time to dinner time. An entire day. Constantly walking on eggshells for fear that Kokoro might snap again and become violent. Once again Kokoro was compared to Kurumi's mother. Always monitoring her. And Kokoro basically planned to become Kurumi's jailer permanently, as Kurumi realized to her horror near the end of the chapter.
It was so sad when Kurumi realized that she couldn't stop Kokoro from invading her house since her mother had given Kokoro the key to do so literally. Also, notice how it was made clear in the panels that Kokoro had unplugged the phone so that Kurumi couldn't call for help again. It also served the purpose of preventing Naoi from contacting Kurumi and being able to explain herself. Kokoro not even wanting Kurumi to be alone for a minute to use the bathroom was really creepy. I hate to say it, but playing along with Kokoro's delusions, even if only to keep her calm, seems to be causing them to escalate. Kokoro has too much power in this situation.
Kurumi is also struggling because of the uncertainty of what Naoi's been up to. Naoi kept the deal with Kudou a secret. And it seems the sound effects in the panel last chapter were Kurumi and Kokoro scrambling for the phone, meaning Kurumi heard the part about the hotel. The thing is though, due to what was happening at the time, it's questionable if Kurumi heard Naoi talk dismissively to Kudou or if she heard the part about the room being part of a "girls night out package", since those are typically used for platonic meetups of female friends. It also seems that current events are likely leading to Kurumi realizing that she has romantic feelings for Naoi, since she can't understand why she's so angry over a "friend" possibly getting a girlfriend.
It certainly doesn't help that Naoi's actual part-time job and drive to earn money from Kudou have kept Naoi occupied lately, meaning that while Naoi wants the money so her and Kurumi can be closer in the future, Kurumi is feeling neglected now. Also, Kurumi's feeling intense unconscious jealousy, which she is having trouble processing as it's likely the first time she felt that way. She even seemed to end up with mental images of Naoi doing "that" with another girl while she was trying to vent by pleasuring herself. They really need to have a talk.
As for the end of the chapter, I have to wonder how Kurumi will respond. Because she just had an angry outburst at Kokoro for letting herself in uninvited a second time. Went off about not caring what Kokoro thinks. And even talked about how mad she was at everyone for being so selfish, Kokoro included. And the first thing Kokoro thinks to say is to ask Kurumi for sex. While I hope Kurumi gives Kokoro a flat rejection, Kurumi's mental state might cause her to initially go along with it before later backing out because it's not what she really wants. At that point Kokoro might try to force things, so that Naoi will get to have her moment in the spotlight next chapter when she disrupts things.
My best guess on the endgame of this volume, is to have Kurumi and Naoi acknowledge their romantic feelings for each other, at least internally. Though I have to wonder where Kurumi will stay after this confrontation ends. Her own house obviously won't feel safe after that day. And Naoi's apartment is likely a no-go due to her father. Maybe they could end up at another hotel. Though if Kudou's still in the area she might see them. Which would be interesting if Kudou also has a sudden realization for her own romantic feelings for Naoi, thus cementing herself as a future antagonist. I'm almost certain she'll end up transferring to their class after summer vacation's over.
This volume started off very heavy, so I imagine chapter 20 is going to have some major catharsis. It might also end up being a pretty long chapter, since chapters 16 and 17 have been relatively shorter.
