Why is Kaoru even dating Hibiki? Was it literally just to take a jab at her ex?
She isn't dating Hibiki. Hibiki wants her attention because Kaoru is the cool, beautiful, admired senpai who picked her up when she was down about being dumped BY HER BOYFRIEND. Kaoru is easily charming and flirts with all the girls around her but that doesn't mean she's dating any of them in the sense of a physical, exclusive relationship.
The only pairs in this series that, at this point, can reasonably be considered as dating couples in a relationship would be Ai x Chie, Shizuka x Mio, Yuu x Mari, and maybe Airi x Michiru. Everyone else is scattered around: from unrequited crush (Nononon), to just friends (Honoka x Alicia), to friends-but-maybe-more (Ayano x Miyoshi), to bizarre alien twin fairies (Mahi-mahi).