If it happens again, I think you should take your son and leave the room, and tell her you can continue the conversation when she is in her right mind.
If nothing else, you've now got a good reason to suggest a counselor to help you communicate.
I think the level of her defensiveness demonstrates that she knows how wrong she is, and also that she's sensing your strength. The unfortunate thing is that she's burning bridges - she could get increasingly defensive because she's ashamed of her own behavior, and blame you for her outbursts. That's why I think removing yourself to another room in the house instantly could be helpful, so she's left holding her own bag.
Another thing is that making an appointment with a counselor and writing her to invite her can help you if she tries to call the police on you later, or leave with S9. I would document this outburst in an e-mail to her with regret and sadness, mention how she must know this is horrible for S9, and ask her to please attend counseling with you to learn how to avoid scenes like this in the future. I know this is not DB'ing, more legal strategy, but that's what I know more about...
M 16 yrs, WH62, P54 3 adult blended kids EA 11/13, BD1 6/14 PA fall 14, BD2 2/15 Piecing 2015, BD3 12/15 Separated 4/16 WH moved OW in 5/16 Divorced 6/15/17