I don't think she gaslit Costley, or that she was seriously asking him to kill Selby. She made a flippant remark about the man who had raped her, which is the type of thing we've all said in anger at times. Costley, who thought of himself as more of a 'hard man' than he really was, decided to get physical in order to impress her. I doubt he actually meant to kill him. In short, I think the verdicts were both correct, really.
If Mark genuinely thought she wanted him dead, then he was covering for her, so he why tell the barrister the sex details? He want throwing her under the bus
He wanted people to know that they had had an affair, to show that he was attractive to her. He seems to have a bit of an inferiority complex, maybe related to failing to get into MI5. He's not all that bright, and I doubt he realised the fact that forcing her to reveal the affair would throw her entire credibility into doubt.