In regards to Akane... on the radio Chii says that she still gets along with Akane and is in contact with her pretty much every day. That's just the story from her side though (there's no reason for her to say it if things soured), I'm not sure how glad Akane is with Haachin who she got along with well and Chii whom she pretty much claimed and got along with right at the beginning when she just entered getting so close to each other. She might feel left out, especially due to HaaChii going to Osaka together. Maybe Akane was busy or something but something tells me they didn't invite her to begin with, which would indeed be gut-wrenching >_>;
There's no helping it if HaaChii feels so strong for each other as friends that they want to do stuff alone ( I know you didn't imply this, just in case it sounds bad). But yeah I get what you mean as in she might feel left out. It's funny to think they might have been in the same gen, maybe HaaChel wouldn't have been a thing if Chii-chan joined with 12ki.
I feel sad for Mai in that way though. I feel Chii-chan was *that person* after Uta left that she considered closest.
But yeah happens to them, happens to everyone, that you meet people that you click with more.