@Kotalier I like your theory, especially because it picks at a new detail, no matter how small and insignificant it can be to others. I would have liked to hear more about your theories on Ichika and Itsuki too, since for Ichika, the only argument thus far I've found have been less theory and more "rough evidence + I think so stubbornly" as much as I want to believe their intentions to be more whole-hearted.
My stance with Miku has been more on the lines of doubt in her being the Kyoto girl because of her attitude which has been the least connected to "being a role model" or whatever adoration for the quints' mother. Doesn't mean she has less heart to it, but at the least from what I've seen, that has been downplayed and the story arcs surrounding Miku have felt more like a journey of steady romance from scratch than an individual who once knew Fuutarou and is starting anew. Some can argue the latter, but with the intro of Rena (and even if there is a possibility that Rena isn't young Rena) I find it that unless the author is pulling big spins, Miku is lower on my list for being the girl. However, among the girls, I'd say Miku stands at Second of being the one to marry Fuutarou... simply from the standpoint of how she is approaching the relationship.
Ichika and Itsuki have been possible candidates, but with lacking evidence in mind. They could be the Kyoto girl, but don't have substantial pointers that are assuring enough. Of course, clues for Yotsuba could also be read as "not substantial" so this may be a subjective thought on how important clues for the two are. Itsuki though feels like she is in the lead due to how many times Itsuki is downplayed as "main heroine" and more as someone who is a friendly relationship with Fuutarou and his family. Plus, the handful of times panels and whatnot have connected Itsuki to the girl (such as when she states "you are necessary to us" as Fuutarou laid on the bed after returning from the forest field trip). But from what I feel with both, the author isn't trying to weave either to be "the Kyoto girl," and if one of them are in the end, the author is attempting a stealth identity with either. I've just haven't been appealed with reasons such as "Ichika is an actress" because as we see with others like Miku, any of the girls could be good at disguises, especially if it's among the quints.
I stick to Yotsuba being the Kyoto girl because among the five, she has had the least assurance of who she is. Of course, many have blanketed her as "the supportive side heroine" but I do beckon people to look further. As
@chocolatedounut has stated, she may be more than she is (as for me, I've detected this all along). The author hasn't made it obvious, but by now, especially with this chapter, I'd wish for people to notice that the author is stating in secret that Yotsuba has a deeper layer to her energetic and seemingly dumb exterior. The lack of book smarts (or is seemingly cuckoo) does not always mean a person is incapable of scheming, although we are conditioned to think like so. But with how Yotsuba has acted, from being Fuutarou's ally from the very start (even for someone as perky as her, she wouldn't just help a stranger like Fuutarou from nowhere, even if in good faith), to the brief moments where she pulls a smile in what some could call "the stepford smiler." Her smiles are not fake, but it's obvious her feelings/thoughts are much more complicated that what people give credit to her for.