Rosemary, like Olivia Pope, has things handled. Long before volunteering to be the lead detective on the case, she arrives at the crime scene to help her son. She then tries to get rid of Margot's car in the lake, but David pinpoints it as an important location early in the case, which leads to the vehicle being found. The detective also creates a false ID for Margot that serves as a fake lead in the investigation.
Then, there's the script that Rosemary feeds Cartoff. She gives him drugs and has him confess to killing Margot in his suicide tape to keep the
case neat and tidy.
Under interrogation, Detective Vick assumes all responsibility, reiterating again and again that she was afraid of what would happen to Robert in prison. Robert, of course, gets arrested anyway.