I honestly think this manga need to get chopped. Once Sora's mom made a deal with her and left for Hokkaido, there is no plot or story that makes the manga moves. So one of course will look at secondary characters being developed. Wakana and Yuina simply are not convincing. Also this happens shortly after Wakana run into her crush. This is totally not realistic the way that arc goes. Then how come Yuina would going out with Wakana supposedly because Wakana and her crush did not work out as it is actually a one sided love by Wakata. For one thing her crush can be heterosexual so the entire arc becomes useless. Yuina and Wakana should have been dating quite earlier. Now it looks like the mangaka does not know what to do, so everything including the kitchen sink are thrown into the story, but unfortunately it does not work,. However I think it is the mangaka's "fault". Whatever happened to Egawa? We never have seen her since the cultural festival.
So who is really the lonely girl? Sora or Wakana or someone else?
I'm not sure I get the criticism here. Sora's mom was the worst part of this story. From the beginning it was really about Sora and Ayaka getting together and figuring out their feelings for each other. There's no reason why a story needs to end when they get together, and there is plenty of space to look at how a relationship develops once a couple has gotten together.
I do agree with you that Yuina and Wakana could have used more development, but I actually think it's better for them to get together after the cultural festival. Their relationship will be better if Wakana has let go of her feelings for her past crush, so I don't mind that, it just happened a bit too quickly, but then again, not every relationship, high school or otherwise, needs to be super serious. They can be more casual about it, and that's nice as well.
And since Ayaka is no longer going for the recommendation, Egawa-sensei isn't important anymore, so would she need to be part of the story at all? She's more of a plot driver than an important character.