If Yuuka IS the primacy force behind non-A days being chosen... she might have had an effect on the situation beforehand because according to Yuuka "it was love at first sight" and her powers clearly allow her to edit away days... so her simply meeting Aya made her decide "let me just edit out anything that keeps her away from me" and gaslight little Aya into reciting everything that happened before hand to create a much shorter period between being hated and meeting Yuuka
this is reinforced by the fact Aya does not ever state or show any clear understanding of WHY she was hated at a young age due to her ability when we see the parents flashback earlier on in the story
This seems to be true regardless of Yūka is the primary driving force of non-A days being true. Yūka puts her thumb on the scale to attempt to isolate Aya.
But, also, they first met when Aya was 10 and, even if Yūka was somehow aware of Aya's power before then, she wasn't in Aya's life so she didn't have the ability to choose days as precisely as she could after meeting Aya.
Also, if she became a witch at a similar age to Michiru, this would mean she had newly become a witch just, like, three or so years before she met Aya and Aya can remember the seven years before that and how non-A days were chosen in those seven years.
Also, this all colors her "love at first sight" narrative. When she met Aya, a 10 year old girl who had just lost her relationship with her parents, isolated and alone and hating herself, but with immense power, she immediately decided that she wanted to possess her. From the start, Aya was always a convenient tool to her. Maybe she also loves her and has been sexually attracted to her since she was 10. That wouldn't, like, make her better, but it's also entirely unclear that this is true.