I used to think that about the basketball arc too, until I realized thare was more than enough there, all handled with subtext. Yuu can be direct with the crazy girl because she knows Maki is there to cool things down if she completely loses it (relying on him with the same total trust she gives Umi). She can focus on training because she knows Maki will find the key that unlocks that girl's issues by any means necessary. She can fight through the first half nightmare because Maki won't let her fight alone (even if she has no way of knowing exactly how).
With all that reliability being aimed straight at Yuu by someone she's already seen quite favorably for quite some time as a person and, for nearly as long, as a guy, it's not surprising her feelings started simmering a lot hotter.