You are right. Honestly I think I would regret not doing my damndest to save my son over regretting treating her the way she treated me. Whenever I've played that scenario out (her raising my son, in her BS chaos and abuse- without me to absorb the evil hate and shield him as best I can) the best I could hope for is he be like yours and 'hate her' (from seeing the truth of her and what went on or was going on).
I really think (from the sounds of it) I would have done the same things as you (at best) and likely got the same outcome (or worse). I've never felt the extreme unhinged anger-strength I felt when she wanted to take my son from me a week ago (she brought it up as I returned from dealing with the outcome of her trying to ruin my career and family interaction/relationships). I remained calm and nice and all the things I wished she treated me, as she screamed and cussed and asked to leave. I was absolutely ready for her to leave and never return, if she was bluffing it was called- what I was also suddenly prepared for, was to do anything to make sure my son didn't become the focus of the abuse I was in. I wasn't going to be amiable for long because of this. If she truly is BPD, it would makes sense, I've never seen abandonment issues in her, she feigns independence (but always had men taking care her needs as she is a hot latina chica). Today when I was discussing her future mental healthcare provider (if she desires remaining in my home- but not thrown in her face) she tried finding the ways to blame me, and point at me being as bad as her... She brought up how much it 'hurt her' that I told her she could "separate" but that I said 'my son didn't ask for separation from my family. Take the truck, get the fuck out, he's staying and you will never take him from me.' She decided to stay. She wants to both have the power and retain her abandonment issues, but they don't work in tandem.
Fuck man.. I'm all over the board tonight. I really had a shitty go of the last week, I've been trying to have healthy shits to get this out of my system after eating nothing but hate and abuse for years. You are likely correct, I ain't arguing with you.