Yami knowing a few days and processing is very different from Yami knowing for a few months, which is what Yuki is picking up on when she says it's all Aya's fault.
Yes, I already said in my post about Yuu didn´t know the exact details of the timeline, and he couldn´t know it. Even thus, Haru seems to think is not Aya´s fault either.
My point is it messes w/ the narrative of "Hikari stole him" if Aya always believed he loved Hikari.
Considering Hikari didn´t reciprocated Yuu in middle school, and Aya knowing Yuu loved her too even if he didn´t leave his love for Hikari, IMO doesn´t mess with the narrative of "Hikari stole him", only reinforces the "without me, she never would have looked him" plot-line
Aya's tsun like behavior is part of a larger pattern of her putting up emotional shields. That's why we still see her deflect - the jokes with Yuu, the doubling down with Hikari - long after she's warmed up to both of them. It's surface level similar, but a tsundere's behavior is driven by embarrassment while Aya's is a trauma response.
I mean, many tsunderes are driven precisely as a trauma response, this is a known fact since Asuka Langley Soryu of Neon Genesis Evangelion. There are several good examples of this with characters like Rin Tohsaka (Fate), Taiga Aisaka (Toradora), Yukino Yukinoshita (Oregairu), Kazusa Touma herself in White Album 2...
And we know Ayami was already thus before the second marriage of her mother (and the abuse attempt), even, if course, Ayami was less tsundere in middle school -according her old friend/admirer we saw in the earlier extra chapter- and more like Normal Kamiya in Shikimori-san
Even if probably Kamiya was a tsundere driven by her own paternal traumas too
I think maybe, like the readers of this manga, they couldn't look away once the trainwreck started.
Hehe.