Our plan for Mother's Day is that my wife and kids and I will spend time with my mother in law, who has tried to help us through dealing with my uBPD mother as much as she is able. My wife and I will prepare and send a card to my mother saying "Happy Mother's Day Mom, just wanted to let you know we are thinking about you today. If you decide that you want to go back to family therapy with S_ (her therapist), we are ready and willing to do that." Did I mention we have been told by my mother via e-mail that she will NEVER go back to the therapist with us?
Any holidays or birthdays upcoming will get an appropriate card with the same message for as long as we continue to mean it. Unfortunately since my uNPD father is silent treating us too we will probably do that same for him. Any thoughts?
I think that card is a really good idea. It walks right on the line of reaching out to her on Mother's Day but also sticking with the boundaries that you've set. You're saying "we're thinking about you and we want you in our lives, but it
has to be healthy so it
has to include therapy," and I think that's perfect. Great idea... .one that I might use in a few months or years if I want to reestablish contact with my mom.
I think you can send it to your dad too. It's similar with NPD, isn't it? As in, you need to set boundaries with them?
Your personal card is a great compromise so that you can acknowledge those special/important days with your parents while still setting out terms for how they are involved in your life. It doesn't have to be that they're all in and wreaking havoc on your life or all out with zero contact. You've found excellent middle ground.

It's true that things may get worse before they get better (they 100% got worse with my mom when I started boundary-setting), but as long as you continue to mean it, like you say, I think it's a good course of action.