I think the bitchiness is just cover for the fact that she's not as smart or together as her glasses indicate. Low self esteem. Then again I know basically jack and squat and maybe I'm just being an apologist because I do kind of like her
I think that's part of it too. I also think she's had her head filled with horror stories about the DIKs (not all of which are undeserved, particularly stuff about Vinny and some about Tommy) and is only just recently beginning to realize that's not true of the MC or all of the DIKs as she had been told.
And while I was joking around earlier, I do actually think Magnar at some point realized Sally wasn't as smart as he thought she was and he dumped her because of that. I wouldn't even be surprised if Cathy actually was tutoring Sally and after losing her job, that put Sally in a spot which is perhaps when Magnar began to realize she wasn't so smart as she pretended.
Also if Cathy actually was helping Sally, then they were perhaps friends. Meaning Cathy wouldn't have paid much attention to Sally in the locker room making it easy for Sally to have gotten that pic unnoticed (at the time at least). Cathy was pretty quick to believe the DIKs didn't post that pic. What if after thinking about it she suspected it was Sally, but didn't want to destroy Sally so kept quiet about it.
It just seems to me Nick was the big red herring in all this, and Sally is the Occam's solution to revealing Magnar's plans to the MC and via the MC everyone else, thus wrapping that up in the planned episodes remaining.
I know I'm reaching a bit there at the end, just thinking out loud. It would bring a lot of things full circle anyway, which is something I like in story telling but I'm not sure if DPC does. Fodder for discussion anyway.