Personally i am quite skeptical this will take an omnibus approach, if for no other reason than such a practice being quite unorthodox (then again, that might well fit with the show as presented) and nothing having been announced on this end. Granted, this show loves surprising it's viewers, so this might well be the ultimate surprise that no one really expected, but i dunno ... i will remain on the fence here.
As for Kazuha and what that would do to her development ... well, i never thought of her as that complex of a character (as Akira notes, she has a quite one-track mind
) and her traits are well established. In a way it is more fulfilling indeed to have her realize that hovering over Akira 24/7 isn't the best road towards happiness for either of them, but that is the solution she tried to push for in the game. While this does put her character development to a halt, so to speak, that doesn't hinder her character from participating in Akira's arc, which is where i expect the anime to transition to (unless they take the omnibus approach ... in which case Akira's arc will be the likely next one as well
). The optimal solution is indeed for her to back off and give both Akira and herself some breathing space, but it is no huge stretch to see her remain as the ever-protective sister, as far as anime timeframe is capable of covering it anyway. For what it's worth, i don't think everything needs to have an underlying "lesson to learn".
Granted, this would make any future interactions and romance prospects for Haruka very, very awkward, so i dunno ... an omnibus would offer a way out from those issues and offer more fulfilling endings for the character arcs, so who knows. I still don't think we would be getting one, but i am open to the idea. As a game player, such an approach caters quite well to me after all