I'm a little puzzled by the 'you-know-who' text. Is this an idiom that is commonly used in Japan? I'd be interested in alternate translations.
I'd like to know how much time Chizuru and Umi have spent together since he was last seen. The majority of the story occurs from Kazuyu's perspective, so we don't see what's been occurring in Chizuru's life. How much does Chizuru talk about Kazuyu when he's not around?
As far as this party goes, if Kazuyu had just confided to Chizuru that he was nervous/uncomfortable with this party (fish out of water situation), that'd offer an opportunity for growing closer, but these two are horrible at communicating with EVERYONE (this, btw, is Ruka's biggest strength. She's the best communicator in the story).
I'm inclined to think that Umi will be the perfect gentleman, and tell Kazuyu that Chizuru turned him down during the summer. Maybe tell him to get his act together or he'll take another chance with her.
But so little is known about Umi that he could turn out to be a cad. We'll see.
But I guarantee that it won't be in the next chapter. This will be stretched out.