
TM Jim, and welcome to the family! I am sorry you are going through this, I am in a somewhat similar situation and it is indeed hard. My take would be that neither of you are ready for polyamory at this stage. The first prerequisite for polyamory is honesty, and it seems your fiance has not been totally honest with you. It sounds as if in the past you have also not always been totally honest with her, and this may -especially with BPD people - take time to repair.
The second is trust, and again this seems to have reached a low point in your relationship. Communication is key, and the problem with polyamory is that another focus takes away emotional energy which you both need right now to put right what has gone wrong.
A big

for me is having the other man in your house. Was this part of your original polyamory agreement? Because it seems to me to be exposing yourself to high risk situations, you do not know much about this man except that he is BPD. Hardly confidence inspiring. Your house is your safe space and you need to be sure it remains that way.
I would suggest dialing back on other partners until you both feel safe and secure with each other. The problem with polyamory in my mind is the practice. The theory sounds fine, but as you can see, the devil is always in the detail.It requires a level of emotional maturity that both parties often do not possess.
The biggest problem with polyamory in relation to BPD is the possibility, in fact probability, of getting caught up in a Karpman Drama Triangle. This happens even in monogamous relationships But with polyamory you are multiplying the chances. People on this board are struggling even to keep one BPD relationship afloat. Here you are contemplating two? You are quite right to be worried.
Here's a good place to start reading about the triangle
https://bpdfamily.com/content/karpman-drama-triangleI am happy to work through it with you because, as I said, I am in a very similar position myself.
Working on your co-dependency issues is a great idea and can only help you going forward. Have you thought of approaching a therapist around the sex addiction? Solving what you can solve on your own is always a good idea.