As I always remind myself when new chapter arrives, it's worth the wait.
Letter drama should straighten the Hajime character, even if with not so positive things.
Hajime overall is a really strong character, she holds a burden, that destroys her whole view of the reality, but still finds reasons to continue. Even though we might say she's running away from facing it directly. Regardless, just the fact, that she didn't break says a lot.
You might say, that she made up an illusion, where she must ensure Momoka's happiness without considering her own well being, but she could've just thrown it all away. She could be seen as someone, who lost hope in life, but I think there are feelings deep within that someday she might be truly happy.
If letter moves plot a way I assume, we should have something like Momoka's breakdown, Hajime's feelings revealed and hopefully her leaving Kyou's shadow to walk her own path, whether with or without Momoka.
Them getting together sounds positive and all that, but it seems extremely complicated. Lets just hope author either makes it thoughtfully and without obvious inconsistenties, or leaves it in some other, maybe kinda dramatic state.
Hajime and Momoka both being in the band with Shiho means that any split or otherwise "hard break" between them risks the band itself dissolving. And neither of them are replaceable by Yori, instrument-wise, so that leaves that issue out.
The biggest issue, is that Momoka still holds onto her love for Kyou. And obviously for good reason--but that letter, or at least the last portion we're given by the author, could cause a huge problem for the group who knew Kyou in life, both intimately (in very clear ways) and "professionally", becuase of the bandmates dynamic.
If Hajime decides to sit on the letter and not show Momoka, then she's got this "my sister wanted to break up with my crush" stressor bouncing around in her head, and it'll resurface every time she sees Momoka in any context. That'll ratched up the pressure on her until she eventually snaps in a moment of strain or weakness, and depending on
that context, could be even worse.
If she does show Momoka...well, that's a huge blow to her, from her girlfriend beyond the grave. Hajime can be right there to comfort her, but Momoka could very well lash out in her pain, blaming Hajime for "ruining Kyou's memory", putting a rift between them all the same.
And that rift risks destroying their band, if even in the short term. They've met with a manager that wants to sign them
now, and I doubt she'd wait for any band "drama" among the members. Even should the two of them eventually reconcile in this way, it would take time, and Shiho's own dream is effectively killed. I can only speak from experience, but multifaceted heartbreak directly affecting multiple group members will
not make for a healthy or productive band dynamic.
That said - there's a chance the body of the letter we did not see, gives a bunch of new info and context to Kyou's wish to break things off with Momoka. It's not like she was sick; she died in an accident, so there's no way she could have known she was not long for the world. Could it have been she knew of her younger sister's feelings? Could it have been she was planning on leaving Japan, and knew a long-distance relationship was untenable? There's too many possibilities, so guessing is kind of pointless as to her reasoning.
But if it's a reason that could push Hajime and Momoka closer together, then there's a chance that Hajime's feelings could result in an accepted confession. Momoka clearly cares about Hajime and they're very close, and I doubt it's solely because of Kyou between them.
I think there's a way through that the author can make this work really well - I'd personally love to see it justified that they get together, even if it's down the line. But I don't really know how it could
not be messy, and potentially jeopardize their band as well.
It might very well take a group effort of
everyone coming together to support them, which might explain why these two are the "last" pair to get their arc.
Everyone else needed to reach a point of stability and comfort, so that their combined emotional scaffolding could help keep Hajime & Momoka from falling apart.