I’m just saying that Hikari never really like Yuu and only started to be attracted to him when he was sad and not a virgin
I'd say Hikari was attracted to Yuu because he adopted a personality more like Yami's, she taught him well, in addition to the heartbreak and losing his virginity. Even in chapters 46-47, Yuu resolves things with Hikari in a way very typical of how Yami used to get Hikari's forgiveness after his mistakes—I mean, Taa-kun picked up on the pattern and knows Hikari well—as I predicted, he would take all the blame and tell Hikari in his typical passive-aggressive way that if she wanted, he would step aside (getting what he wanted, for Hikari to make the opposite decision and choose to maintain her bond with Yuu even if it could cost her her friendship/relationship with Aya).
Sure, but at that point he already had a good idea in his head of who the characters were - it's in their names.
That and their introduction. There's a reason Yami's debut episode featured her trying to steal a kiss from Hikari. The plot twist wasn't so unexpected; what was truly unexpected was that she already had such a deep and complex past with Yuu, a past they both managed to hide so well.
I think the suicide attempt makes sense for who Yami's mother was already written to be - clingy, unstable, desperate, so desperate to not be alone she shifts from partner to daughter.
Even so, it only makes sense if Yami's mother somehow found out about Yuu, either before or during the trip to the beach hotel. Only that would explain such a clear cause-and-effect chain in Yami's case. And it would make sense that Yami's mother would use the same detective Yami hired on her behalf to prove her stepfather's infidelities to spy on Yami herself and see who she was seeing.
That, and of course, any minimally observant mother would have noticed what even Yuzuka could detect without much trouble.