The only wrong thing would be to deny what your heart truly feels.
I think it’s possible, yes. I personally think it’s Alison who did it, however, Marlene stated after the premiere back in June that CeCe’s storyline was not over, so it’s absolutely possible CeCe killed Mona. It’s also possible that Alison had CeCe kill Mona. Many possibilities!
Black Widow was first seen in 4x01 at Detective Wilden’s funeral.
She’s made very, very few appearances since then, that we are aware of. She is supposed to be the main villain of this season. It’s apparently a character we already know, and supposedly, we saw her unmasked at some point in 4b. She is (I think) also “Uber A”. She is the leader of the A Team. She’s a bad, bad lady!