For example, many of them seem charismatic, engaging, etc. to people they are not romantically or intimately involved with (in a platonic setting). When it comes to their SOs, though, things are obviously different.
Are pwBPD aware of this? Is there a conscious effort to switch? Or is it all unconscious?
I think it comes back to their lack of self-identity and insecurities. My ex used to put on a face around different people. Her own friends, my friends, my family. She'd tell me this. If she was invited to one of my friends' bridal or baby showers, she'd tell me that she's only going because the girl is my friend and she'll put on a face until she leaves. She'd put on a nice face in front of my parents, then as soon as we left she'd pull the mask off. I think it's something they're fully aware of because it's how they've been able to survive, but I think in their mind it's totally justifiable.
Now that I think about it, she did used to talk about "survival techniques." I'd try to explain to her sometimes that that's not how the world really works. It's like it just wouldn't compute, and she'd toss out any justification just to prove herself. It's like trying to discuss Shakespeare with a tiger.