what's happening with her hasn't been relevant to anyone except for her. While all the other sub plots intersect with each other
Sort of, but not quite.
Consider that the other plots were much more isolated, prior to these more recent episodes, so it's not particularly strange that her subplot would be the same. And then remember that her struggle figuring out she's into girls was the smallest and last-acknowledged subplot we've gotten, so it just seems more off on its own--though, yes, the only one it
directly involves is her (she had that disposable dude try to give her struggle some actual character interaction, but...well, I mean, I just described him as disposable, so....), even as this has been growing in the story for about as long as the Fading Beauty/Izumi stuff.
However...once things started to overlap (and, really, the Glasses-senpai/Erotica-chan overlap only
just happened), Twin Braids Girl's struggle should definitely have been woven more directly into the Kazusa/Izumi/Fading Beauty story.
I think the misstep in all of this, of course, is Pedo-sensei. He's a plot point we don't need to explain Fading Beauty's actions--AND--Twin Braids Girl could easily be Fading Beauty's refuge, after she loses Izumi to Kazusa, making all her "supportive advice" a double-edged sabotage as she tries to both be a good friend and seduce the object of her desire.
This way Momoko has a slightly better idea of [how] alienated she is from her friends.
Some people are just awkward.
I think that's being generous--to the writing, that is. Because I think you're right that Twin Braids Girl is meant to be awkward and alienated--and she definitely is, logically.
But if that's what she's meant to convey in these moments...ehhhh. I mean, she'd be more upset or nervous or withdrawn than she is. She'd stumble over and/or reprimand herself for saying things that come out sounding bat**** insane when she's really just having a hard time putting her feelings (which are new and confusing) into words. She'd be sad and embarrassed and lash out at odd moments, not act alternately sociopathic.
She should be coming across--or, at least, clearly
supposed to be coming across--as someone who is trapped in a situation she doesn't yet know the way out of. We should be able to look at her and
see her struggling, not just interpret or intellectualize that she's struggling.
And, for the purposes of dramatic writing, she'd have asked a better "would you rather" question. Or a more pointed rhetorical question about finding her attractive or choosing to be with a girl if all the boys died or something.