I'm sorry to hear of this turn of events Yellowbird.
I've heard of those cultish "therapists" too who recommend severing ties. I'd be extremely skeptical. None of my therapists would have told me to cut contact with someone, even my abusive parents. They discussed all angles with me, yes, but never did they give advice.
To me that advice is crazy. You're his main support system, he is recovering from a severe trauma, of course he is upset and it figures he might lash out/push away anyone within a 50 foot radius. Removing everything in that 50 foot radius isn't the answer. In fact the answer is the opposite, trying to keep our support system intact WHILE ALSO working through the trauma, if at all possible.
I was where your fiance is, up to my neck in PTSD and triggers regarding anyone getting remotely close to me, and my instinct was to push my partner far, far away. I went to my therapist EVERY WEEK for months asking if I should get divorced. In retrospect, I was 80% reacting to the trauma and 20% to the relationship (and she eventually helped me see this).
I can only imagine the pain that you must be in watching things unravel like this. It sounds like you are trying everything you can. Your therapist sounds like a level headed and wise person, given the situation your fiance is presenting you with, perhaps putting the relationship on hold in your mind would allow you to protect yourself from further hurt.