Baby Momoka (well, 6th grader Momoka) is giving such big Himari vibes that I can see why they clicked so quickly upon meeting one another.
Finding out that Hajime had fallen for Momoka even before any confession to her elder sister happened, though, is heart-wrenching. And yet, even after "losing" Momoka to oh-so-perfect Kyou, Hajime wasn't pushed out of their group. And Hajime taking that in stride and finding the happiness in their trio is heartening, because a lesser person would have self-destructed and "been considerate" only to build resentment.
Which would have surely broken Hajime when Kyou died. Being able to carry on with Momoka, both licking the other's wounds with gently-forced distance, worked for years; and as close as they were, even with the wall of lost loves in multiple directions...at least it was a cold peace.
I don't think Hajime will ultimately regret the finding of that letter, or the spilling of her guts to Momoka. In the moment, the pain will be immense, and the ramifications will be drastic and dire where their band is concerned and the fabric & texture of the friendships they share.
But they have friendships, with a group of girls who care about them both to a painful degree. Time will be required, and more than one difficult conversation...but I don't see Hajime and Momoka falling apart. Even without Kyou physically present to underpin the intertwining of their lives, her memory is something indelible to them both, and while she never knew Hajime's feelings, she also had wished that Momoka find happiness without her (though not intending it to be so permanent).
It's gonna suck and hurt, but I have faith they'll come out the other side intact and able to look forward.
Thanks for the TL.