The last few pages of chapter 4 gave me chills but I do agree that this is probably just a metaphor. I think this was made to emphasize the theme of Tanabata, as in a tragedy, where characters are not in control of their destiny. The answer of Erika is also intriguing, as if she was relieved.
But so far, nothing indicates that Koto is manipulative. If we exclude the last pages of chapter 4, Koto just seems a bit confused and kind of dull. The gap between Koto and Aya is blatant though and somehow, it looks like Erika is connecting more genuinely with Aya than Koto (this is true in the following chapters as well). Mostly because she is able to be her true self with Aya, while Koto seems to have shut downed her feeling 7 years ago, making it difficult for Aya to connect with her. At least, that's the way I see it.
It still don't now what the purpose of the story is, nor how the relationship with those three characters will evolve, but this is one of my favorites lately. I really like the slow pace of Kabocha's stories.
Btw, the first volume was released as an ebook only, no physical copy. Try to support the author if you can, I don't think their are selling a lot (though, chapters are being released on some webcomic plateforms as well). I do hope there are enough sells in order to justify a physical release at one point.