Hi pandadoll,
I would prefer to not talk to her. I realize that I must on occasion. If I don't have to then I prefer not too. My boundaries with her conversations strictly about the children. I do not care to nor will I discuss my relationship with my husband with her. Her relationship with my husband or my relationship with her. Too many times I have had the "slap" in the face for allowing any of that to happen. I will not be friends with someone who behaves the way she does towards me. If it were anyone else besides my step childrens' mother I would have long severed any type of relationship we have.
This is how I feel as well.

The pwBPD in my life and I have a very good
relationship. I say relationship, not friendship, because I would not be close friends with her, not necessarily because of
who she is but mostly because of
who I am.
I've come to value myself very much and I just don't invest in friendships [anymore] when the other person doesn't want to share in valuing me as well. The mama of my stepkids just really struggles in personal relationships and that's OK. I won't struggle with her in that capacity because being in a friendship with her takes a lot of give and not a whole lot back and it's just not something I'm interested in.
Not a reflection of her, just one of me.
I don't have to go outside of who I am in expressing that to her either. I still treat her with kindness and respect because I am a kind and respectful person.
Usually I do not communicate my boundaries with her. I just act upon them. In this circumstance I felt it was necessary to communicate that. She is the type to blow the door right down if you open it just a little. I am allowed my space and privacy.
I think that you are doing a good job of keeping yourself protected from what a disordered soul can bring into the picture. I ask about your boundaries and what kind of relationship you want with her only because your expression/acting on your boundary setting is relatively aggressive - sometimes it needs to be. Sometimes it doesn't though, we can still express our boundaries with an
"It works better for me when we x, y, z" rather then
"I will only x, y, z and if you try to do a, b, c, - I will respond with q, r, s." Boundaries are about
us and protecting
our values.
I, too, have a very strict boundary when it comes to communication about my stepchildren/children with the other parents in my life. It's more about content then anything but it's about protecting my value and implementing the boundaries in order to do so.
Value = I believe that communication about important issues regarding the kids should always stay between the biological parents.
Boundary = Other then with my husband, I don't partake in conversations where I am not the other half of the bio-parenting pair
i.e. I won't discuss important issues about my own children with their stepmom and universal with my value, I won't discuss important issues about my stepkids with their mama.
When Mama has tried to drag me into an issue, I express my value/boundary "Mama, I get what you're saying but you know I won't go there. You need to talk to the hubs about it."
And she usually responds with "I know, I know".
Me:I prefer to not have communication with you. I am willing to allow an open line of communication by TEXT only in matters regarding the kids. I will not answer the phone if you call.
It's why I ask what boundary you are setting? What value are you protecting?
Her kindness was met with an aggressive stance on your part. (No judgement on my part, just an observation) My guess is that the hurt runs deep in your situation and you just aren't ready to place nice yet. I get that. I didn't talk to the pwBPD in my life for 8 months in order to be in a different place with her. My cracking the door back open started off with an apology on my part and lots of boundary setting when she barged right in.

We just have to be cautious when we are reacting/responding aggressively. It's why I ask what would your relationship look like if it was just the way you wanted it?
Would it be business-like but respectful?
Would it be texting back and forth about the drop offs/pickups in a friendly manner?
Or would it be where each of you has a day here and there where you feel like you are "winning"?
What is the best scenario for the kids?
~DreamGirl