But we are in a manga, so the scenario is probably not going that way The third one seem unlikely for now, since Kaoru never really showed any indication to be into girls. But that could change.
I think, only last of three scenarios is possible here. First two just kills Kaouru as a MC in the manga. But I agree that now Kaouru absolutely not into girls, so I'm curious how mangaka will trying to fix it and how long it takes.
IMO, the next move of the author will be to make things explode harder, with the cheating of Reiichi being exposed. Before or after Uta leaves. If it's before, Uta won't leave, because Kaoru will be devastated and how could Uta leave her love interest alone? Or she will come back running.
Yeah, and here we have another interesting possibility, when Uta somehow supress her romantic feelings, takes responsibility on Kaouru, and they both start living as a family. So the manga ends right at the point, where "Ani no Yome to Kurashite Imasu" was started.