Got it.
Melissa was being blackmailed the whole damn series, and even now. I don't think it was Charlotte because this person knew that Melissa buried Bethany alive and killed her, and Charlotte didn't know that.
So the blackmail started with the Black Swan crap, the Halloween train, Melissa breaking her face masks. "I've been protecting you since before it started." Before WHAT started, Melissa? The blackmail? Because that wasn't CeCe. And the person is continuing the blackmail even 5 years later. They ruined her relationship with Wren and are getting money out of her. No wonder Melissa was asking about the tape. No one else saw the tape, but someone else knew all these years that Melissa did it because they probably saw her do it... or Melissa told them that night.
What if she thought she was talking to CeCe and she told her she buried Alison (which she thought at the time)? But it was really the twin/doppelganger, and that's how they've been blackmailing her this whole time. And that's why Melissa thinks it was Charlotte who was blackmailing her all these years (including calling Wren and telling him about her killing Bethany), but since Charlotte died, Melissa has no idea how someone else besides Spencer knows. That's why she asked about the tape immediately when she came back. And we know it wasn't Charlotte because in 6x10 said that she didn't see Melissa that night. And that's true, because she was in all black while the girl talking to Melissa was wearing the yellow top.
I keep wondering why exactly it is stated over and over why they need the killers name before the election… And then I realized, the election is a deadline. If Melissa doesn’t find out who the killer is before Election Day, the killer is going to release the video of her confessing to Bethany’s murder. The killer is blackmailing Melissa, and Melissa is torturing the girls to figure out which one of them is doing it. But that’s not all, whoever Charlotte’s killer is (Rollins/twin/doppelganger) is also blackmailing Mona for the murder of Bethany, and is using it against her to frame Melissa for Charlotte’s murder as well. Melissa thinks the girls know who killed Charlotte, or she thinks one of them actually committed it, in turn - since Melissa is being blackmailed for Bethany’s murder, and Melissa is pushing the girls to confess, the actual killer is going to get away with it. Either Melissa is going to be framed into being the killer, or one of the girls/Mona will go down for it.
So, the killer (Rollins/twin/doppelganger) is blackmailing both Melissa and Mona, is forcing Mona to frame Melissa, and Melissa is torturing the girls to find out who the killer is before the confession video is released. And at the same time they're trying to make Mona look guilty since she made that phone call. And the killer is just sitting back and watching it all happen.
So, the motive probably links back to Bethany's death. The twin is probably Bethany's twin/a doppelganger of Charlotte.
