bbm
You said it.
Will CW get up on the stand and tell that story about his telling SW that he wanted a divorce and that it put her into a murderous rage?? Oohh, if so, the prosecution will rip him a new one -- I don't think he is bright enuff to take the stand without screwing it up, and I feel sure his defense team will easily notice his limited smarts sooner than later.
So, IMO, the only way the jury hears that CW asked for a divorce would be: 1) opening/closing arguments (not evidence), 2) a psychologist could relate what he told him/her in a session, or 3) a friend of his might testify that he told him/her about it, but then the question would be when did he tell his friend.
Further, it may be that SW told her parents that she was thinking about a separation/divorce on her trip there -- she was probably confused, sad, mad and upset. Her parents would surely take the stand to contest his story about it, if necessary.