I wish I had the answers as well. We have a BPBM (Borderline Personality Biological Mom) as well. It's very difficult. We're still in court, but here's what we're doing:
1. Set firm, clear boundaries. These people are living examples of "give them an inch and they'll take a mile".
2. Document absolutely everything. My suggestion is that you record all your phone calls. If your state is a two-party state, then every time you start a conversation with the person, state calmly and clearing that you are recording the conversation for your records. They can either talk calmly in a way that they would like a judge to hear, or they can hang up. Both options are better than getting yelled at.
3. Do not share any more information with them than is absolutely necesary. They are manipulators of the highest order, and will use everything they get to inflict their misery onto you.
4. Make a "BP Free Zone" in your house. That means that, when you are "in the zone" you do not discuss the BP. It's really easy to get caught up in all thier craziness and let it disrupt your life. You have to talk about it sometimes, but not all the time.
5. Ask the judge to assign a parenting co-ordinator. This person is usually a qualified therapist who will help mediate between you and your BP. The BP will most likely stop complying with them after a short period, but then you have a witness to thier behavior.
6. If your BP has been diagnosed, ask the judge to require them to attend individual treatment. If they refuse to attend treatment, ask for
supervised visitation.
7. Have a detailed
parenting plan. This means that everything it spelled out. Instead of "weekend visitation" it should say "visitation commences each Friday at 5pm and concludes Sunday at 6pm". Days and times for phone calls should be in there, transportation, and a conflict resolution plan should be in there as well. This way you have a limited need to communicate with the BP without it interfering with their ability to have a relationship with their child. This way everyone knows when visitation is, how they will get back and forth, etc, and there's no need for discussion. This is hard for the BP, and they won't follow it, but it gives you the option to ignore their attention seeking behavior without looking like a non-cooperative parent.
8. Don't try to "figure out why they're doing this". You can't, and you'll just make yourself crazy trying.
9. Try to limit how much influence they have on you and your life. Set a good example for the kids. Be healthy. The kids are going to have to learn how to deal with a mother who is mentally ill. That's a huge challenge. Expect that they will struggle with it. Help them develop the tools they'll need. There's no way to protect them from it, so you might as well let go of that wish and help them learn to deal.
If anyone else has suggestions, I'd love to read them!