okay so what I'm reasonably sure of is that Mami didn't get bullied the way she did. Instead, the thing is she feels like she can't connect with people and the way she acts always drives them away, so she tries to get every last drop of pity and sympathy she can out of them before they decide not to be with her anymore (or she decides she can't be with them, because it's not giving her what she needs) and she leaves, and the last time she did it she spread rumors to get in with her new group.
Meanwhile, it seems Taka has her own sort of unhealthy attachment issues: she gravitates toward pitiful people like Mami that have something wrong with them and need her. I think Yuki was the previous person she did this with, given that she seems to have strong insight into how Taka works and knows that Taka'll leave them because there's nothing wrong with them, and it seems she's hit with strong feelings of abandonment. Taka has that sort of deadpan expression because it looks like she had managed to get somewhat stable with Yuki and her other friend, but seeing Mami the way she is she sort of relapsed to her old habits. That scene where she takes their hands, is her saying goodbye, and that she really wants to stay with them but she's already lost to her attachment issues.
I think this is the most accurate take. Mami and Taka are people who look for validation from others, albeit in different ways. Mami looks for pity and attention, while Taka looks to "save people" hoping they'll be grateful. I guess this is about the two finding their perfect match.
Personally, I like Shiho. What she went through with violin is a very real issue. If you've built your entire identity around being good or "the best" at something, it can be incredibly jarring and demotivating to see that maybe you aren't all that. It actually happens a lot in the scene of classical music, musicians will get so much anxiety from just seeing their instruments that they have to stop.
I don't think any of this excuses her behavior, but it does explain it, and I can sympathize with her.
Also, this clearly isn't the only reason she left SS Girls, I'm not sure why some people are treating it like that.