He may be on a courtship now - but unless he learns to actively manage his issues somehow - then you know how the courtship ends.
My Ex has Fibro/Chronic Fatigue. She has also had anxiety attacks, PPD, depression, and was briefly labeled as BPII years ago. So many of these symptoms overlap - including BPD. Even Fibro - which is supposed to be physical - looks just like depression when you lay the symptoms out.
When things were bad, I asked her once if she knew what a Narcissist was - she raised her hand and smiled. Truly creeped me out.
As for your son - I don't think you can really do much - except continue being the stable parent. My Ex and my oldest D can't stand each other - and it is really hard to not side with my D sometimes. But - I continue to send the message to her that her Mom is her Mom, and she needs to listen and be respectful.
If it ever crossed over to abuse or neglect, then things would change. But the sad reality is that her Mom is just not always a good person - and both girls are going to need to find some way to deal with that.
And - I guess it is the disorder - but also the way your STBX has (or hasnt) dealt with the disorder.
I know you miss the good times - but imagine that same kind of relationship without all of the "bad" stuff! It's out there!!!
“Fighting for peace is like screwing for virginity!" - Carlin