Except it wasn't physical, which makes all the difference. To call Koto's behavior abusive, in my own definition, it would need to be a repetitive behavior. As a one time thing, it's out of line and not acceptable, especially with the age difference. But an isolated incident is not an abusive behavior, at least for me. And in terms of storytelling, the point of that scene was to show us her breaking point, not to show her becoming an abusive person. With this story, I feel like people are too quick to call behaviors as toxic, abusive or crossing a line.
Without going into details about my personal life (not interesting anyway), the reason I'd like to see the Aya/Koto relationship works is because I once was in Koto's shoes. Some past trauma made me possessive and jealous, to an extent it wasn't healthy. And the same as Koto, I wasn't aware of my own trauma.
The reason I was able to get past it, understand my own trauma and work on it was because I met someone that loved me enough to put up with my shit and see through it. If it wasn't for that person, who stucked with me while I was processing all of that, I don't think I would have been able to even be aware of my issues. Funny enough, I was around the same age as Koto as well when it happened (and that person was a bit younger as well, not as much though).
And while Aya is young, I definitely think she can be that person for Koto, while Koto supports her while we can finish her study and becoming a proper adult in society's eye. If they can make it work, it could end up being a strong and lasting relationship.
Erika on the other end doesn't seem fit for that role. She is too focused on her own feelings to be able to give the help or even just the push Koto needs. I don't hate the character though. Overall, she is still doing a lot for Aya, and while she does have ulterior motives, which makes her feels pathetic and like a bad person, she is still doing a lot for her regardless. If it wasn't for those flashbacks, and being able to read her thoughts, I don't think anyone would think badly of her. I don't see any of her behavior as actively trying to sabotage Aya/Koto's relationship either. That's what she hopes, but not how she behaves. Even asking the question "do you really love present Koto" is legitimate. Aya never questioned her feelings for Koto, and it's definitely necessary for their relationship to work properly. And yes, I know it's not what Erika is wishing for, but it doesn't mean the question is out of line/crossing a line.
tl:dr
While not behaving at their best, I think all characters's behaviors are relatable and understandable. This whole story seems to be written to allow those characters to go past their own flaw and trauma, in order to become better persons. That's the way I see it.