You are in a tough spot. I'm sorry for all the pain you are experiencing. Don't feel guilty about fantasizing about living separately in peace -- given the load you're under, it would be surprising if you were not having these thoughts. They are normal.
A quick point about your wife's "triangulation" argument. Don't take what she says about what happened in therapy as gospel. It's possible that her account is affected by her feelings and may not be an accurate representation of what happened in therapy, or her application of what was discussed to the situation that night may not be what the therapist intended. Don't confront her about it. Just take what she said with a grain of salt and try not to be as threatened by it.
You talked about a "boundary" that a family discussion can always happen. Adjusting your understanding on boundaries can help avoid a lot of grief. To learn more about boundaries, visit this page on
setting boundaries. Boundaries are actions we take to protect ourselves, like leaving the room when someone is yelling. I've caused a lot of conflict by expecting what I thought was "healthy" behavior from a pwBPD. Nobody wants to feel controlled, particularly a pwBPD. I found the approach that works best for me is to think of it as giving an invitation to healthy behavior, and if they don't accept, I may be disappointed, but not indignant, and I just let it go.
Some of your wife's behaviors towards you and your son are concerning. You may wish with all your heart that you could change her behavior, but it's not likely to be successful. You
can have some success learning coping tools here to reduce conflict levels (for example, adjusting your concept of boundaries). These changes alone may not get things to the level that you want, but you'll get to a better place and you can think about the future from there. Does that make sense?
RC