Erika remains the most interesting character in the series. She reminds me of Saeki Sayaka in many ways but less repressed, probably due to being an adult. So far she's doing everything right, and incredibly enough her wish on the tanabata seems to have created the story as we're seeing it today.
People are trying to find a villain to blame here, but neither Erika or Koto are really bad people. The title "how to break a triangle" is extremely apt here, because that's exactly what Aya's disappearance did to both of them. Koto was in love with Aya, and recieved a positive response from Aya too, and then she disappeared. What should have been a typical high school romance stagnated and froze in place.
There's no way of knowing if Aya and Koto would have even been happy together, for all we know they would have broken up within a couple months. But because Aya disappeared that those feelings sat and stagnated for 7 years, and became very twisted as a result. I don't think it's really accurate to say that Koto loves Aya, more like she loved Aya, and without any outlet for those feelings this entire time, it's become more of an obsession or dependency issue.
I don't know if it was really correct but I do think Aya coming back is kinda the best way to force Koto to process and get over those stagnant feelings, although it will likely be at the cost of Aya being really miserable due to her unavoidable circumstances. Compared to Koto I think Erika has done a much better job processing her feelings towards Aya and Koto, although her love for Koto has been in stasis, I don't think it's necessarily gone sour in the same way Koto's feelings seem to have gone.