I can't disagree with 2010.
Once a 3rd person is put in the equation, the dynamics of triangulation
(read definition) kick in and the balance changes.
Another thing that comes to mind from your description, Ugh, is that we tend to attribute a lot to the mental illness itself when, in fact, pwBPD repeatedly show that there are plenty of instances where they could react differently, yet repeatedly choose not to. It's hard to tell where the mental illness stops and where free will (or the true person behin the illness) begins. I can no longer separate the 2. I think that they are driven by an illness, but are often highly intelligent and excert control in the ways that they choose (otherwise they would have sought out help, wouldn't they?). Plus the manipulation tactics towards us as partners wouldn't work if they were all out there in the open for everyone to witness. Though, having said that, my exBPDbf did throw some pretty low blows at me in front of his parents in a passive-aggressive manner (aka in the name of humor - NOT).