So... .I basically just need to work on her behavior.
How? Her behavior is not something you have the power to control.
What kind of consequences are effective and appropriate for BPD misbehaviors?
I think it might be more helpful to frame the question in terms of what are effective ways to take care of yourself and your boundaries. You only have power over yourself. You can choose to act with integrity to protect what you value. You cannot punish your mother-in-law into behaving the way you want her to. This workshop might be a good place to start looking at things in that light:
Getting Our Values and Boundaries in OrderLet's take the example your husband offered of leaving when she misbehaves during dinner. Here are two ways of looking at that action:
1)She's mistreating me. I will make her experience a negative consequence to her negative behavior so she'll feel bad that she did it, and maybe that'll teach her not to do it again.
2)I value respect, and I am worth respecting. She isn't respecting me right now, so I'm going to take care of myself by going somewhere I can be treated respectfully.
In both cases, the action of leaving is the same; the intent behind it is what's different. The first thought process, at the heart of it, is manipulative and punitive. And it's likely to increase the sense of conflict felt on all sides. The second is self-oriented. You can decide what you will and will not allow in your life. Does that distinction make sense?
I can tell you feel ignored and unappreciated by your MIL. That hurts. You want her to acknowledge that she's mistreated you and apologize. If she has BPD, you know it's not likely that's ever going to happen, TDeer. But you do always have the option of confronting her about the conflicts you've had or the way her behavior makes you feel. Just remember she will have the skill set of someone who has BPD, so she will behave accordingly. I recommend using some of these
Communication tools (SET, PUVAS, DEARMAN) if you ever do decide to try talking with her. Sometimes I have felt I need to engage in healthy confrontation, if only to express that I have needs and feelings, too; at other times, I find it better to move on without it. It's up to you in each situation.
All of that said, I see that you're wondering if it's even going to be relevant or if she'll give you the cold shoulder forever. Maybe she will; or maybe her mood will change and she'll act as if there was never a problem. It's not predictable. You don't need to sort out every possible contingency ahead of time. That's exhausting because there are so many. But if you take a look at your values and boundaries, and discuss your family values together with your husband, you'll both know what's best to do in any case when the time comes. And not just in your relationship with your MIL, but in other areas of your life as well.