To stop the panic let me say it: Kagami making Shinoa love Yuu is fine but Yuu doesn't love her back. Because they have no romantic chemistry.
I guess to divert our expectations they had yuu stay with Mika a lot but they have more chemistry now because of it. Hell Mika and Yuu dedicated a big time in the manga to one another so it would be odd for yuu to suddenly fall in love with Shinoa.
They have more of a close friend or brother and sister relationship then would be lovers. Shinoa obviously had feelings for yuu but unless yuu's naivety and obliviousness is that damn strong he never noticed she felt this to and tried to ask her out in the years they knew each other, then he doesn't like her that.
I understand why they wouldn't as we all have been shook to the core but no chemistry means no healthy relationship. What will they do? Shinoa and yuu rarely hang out by themselves if at all anymore, Mika will probably loose his shit, Yuu is way to dedicated to Mika to even notice or acknowledge her feelings and hell Shinoa never even tried to open up to him about it.
Flashbacks wouldn't work since it would make us question why they don't act like that now and yuu would seem like a douche pushing her aside (which he kinda did with the others...) For Mika which was adorable but cruelish.
And first loves aren't common. Yes it's a trope used in anime but realistically she can actually just have a crush and move on after a while. Shinoa is pretty hot and I'm sure she can find someone else.
