Let's admit. We all saw this coming from a mile away. The relationship between the three is,... fragile. The childhood friend status quo would have made good fortification for the trio but after watching the end of S1, I feel like the only reason they're even still friends is because of Honoka's selfishness. If one of them would try to make a move on Honoka, rather than being happy for them, it's quite likely that the other would not sit well with it. Yandere Kotori is indeed realistic if the relationship between them broke. If Kotori were to break the status quo, then Umi would probably stop hanging out with Honoka altogether, because she can't put on a fake smile like Kotori, and both her and Honoka are far too different.
This doujin kinda brings you to question whether or not the Childhood Trio is indeed what it seems. They've been through hell together and, like Nozomi coined, they've seen each other in their most embarrassing state. In Kotori's case, though. It might have been horrifying instead.
That's probably one of the reasons I don't want to root for the Second Year OT3. I mean, you can make the Third Years or even Second Years an OT3 and it wouldn't even be hard to imagine. The Seconds Years however, what with how different their personalities are, wouldn't last long. Umi's personality is far from the others' and she's probably the one who can keep the others from going insane. Kotori's pretty much what you guys call a Yandere. She won't be able to handle sharing her beloved. Honoka? She's the most destructive of all. Being a good samaritan isn't as fun as one might think. She takes all the blame herself, she doesn't look back at what the others are doing, only pulling them ahead, she is good to everyone, thus not realizing what her good deeds will bring to others. Kotori and Umi probably knew that.
All in all, this doujin questions the dynamics of the Second Year Trio and I'm really hoping the author would make another one with the same elements. These dark stories for 'good and loveable' fandoms are a blessing to us all.