I didn't say anything about Emma gets raped by Zak and im and i feel uncomfortable about the direction the thread is going. I personally believe that Emma gets talked, naive as she is, into it. And even is she just the bonus cherry - what a pun, ha! - on top to the way of Charlotte.
This!
If DB wants to get a good balance between closing this part of the story in a believable way and preventing Charlotte and Emma from suffering too much while he keeps the tension till the end, he can split it in two sections.
In the first one, during the party, Zak tries to force Emma and somehow she manages to escape from him. Maybe someone goes so see what's going on in the room where they are after having heard Emma screaming. This is where the image of Emma leaving the party crying comes in.
However, a couple days later, he phones her trying to apologize, saying that he made a mistake, blaming the alcohol. Our naive Emma gives him a second chance maybe thinking that she was somehow responsible due to her lack of experience and, as a result, he finally has the opportunity to get into their house.
After that he just needs to wait for the right moment (when the MC is not at home) to drug them both (Emma just gets out of the equation) and, when he already has Charlotte naked on the bed and has just started to touch her (with the cameras already recording), the MC goes back home right after having met the detective and gets into the room just in time before Zak can go any further. Here is when the MC crushes Zak, and now they finally have all they need to close the case and to convince Emma about how Zak actually is.
Regards