I think what we need to accept about these last few chapters is that both sides were in the wrong and that this is fine.
Kase-san may have acted possessive, but that's nothing new and her fears and concerns are understandable. Especially with Yamada being the way she is. The fact that she just hung up on her also probably made her feel especially guilty about it for that week (heck she thought maybe Yamada broke up with her because of it). The real problem here is something else though. The fact that Kase-san does everything she doesn't want Yamada to do, that she wants to be kept informed on all these things, but doesn't do the same in return. She didn't tell Yamada about a one-week trip without contact until the last second (and that seemed to barely have crossed her mind if it hadnt been brought up just in time). She also went to exactly this kind of place herself with her club before, so that certainly reeks of hypocrisy, even if we know she is more faithful to Yamada than a loyal pet dog.
Yamada feeling jealous and left out of the loop is completely understandable and her short-circuit reaction was not completely unjustified. Of course she felt guilty about it too and even thought that Kase-san would break up with her over it (same insecurities between these two), but she still chose to ignore her partner's concerns and fears and bulldoze through it, which is never good. While Kase-san probably understood deep down that this is not the kind of thing Yamada enjoys, she still has to accept however that if a person never steps out of their comfort zone they won't learn new things to enjoy. Yamada didn't do it to spite Kase-san initially, but just to do a friend a favor.
They both overreacted and they both did their best to make up for it.
PS: On the topic of mixers, the general idea is that you have a group of mingling singles, sure, but often many members are just supplementary to have a better mood going (e.g. it's easier for girls to loosen up when they have other girls around them). There are clearly available and non-available members in the make-ups most of the time. And it's more than likely that some will not hit it off with anyone and that's expected.
Now this place clearly isn't a traditional mixer and the policy of "seperate from your friends" is pretty shady to say the least, but in the end nobody has to do anything they don't want to. Yamada wouldn't have taken any alcohol anyway and as we saw, her friend would have dragged her out soon even if Kase-san never went there. It's that kind of trust you build in new friendships.
PSPS: First names are still these girls kryptonite and I'm melting away~~