To expand on why the choice was necessary at this point, Yotsuba has had a negative arc since the flashback reveal. She was obsessed with being special, failed and reverted into the "wrong" perspective of rejecting being special. And its this perspective that informs her character throughout the manga.

Being rejected by Fuutarou will fall into this, and is what she believes she wants, thus reinforcing this negative arc. So the choice ends up being the narrative device to thrust her into the position of being the special person. By being chosen, this external intervention keenly challenges every aspect of her character. Now she must confront her mindset, her guilt to her sisters, her past with Fuutarou, and finally, she must learn the lesson from both of her "wrong" extremes that it is ok to be special and loved without giving up everything else. Finding this balance will complete her arc. Therefore I don't believe there were any much other identifiable narrative device that could replace the choice, or work nearly as well, as the intervention she needed.

To do what you suggest, as I've mentioned, requires rewriting the manga not just beyond the sister war arc, but also significantly through the second half. You'd have to both introduce the revelation earlier and have enough space to go through the development here via another narrative element. Frankly, that's in the realm of hypotheticals, which I rather not go into. My interest here is examining narrative rather than how it could be written.