Yes, Kate, it appears you are in the throes of N-ism. Your mother reminds me very much of mine. But what spoke to me more what how I identified with YOUR reaction. I can see myself in you and have walked in your shoes of guilt many times.
I cannot honestly tell you how to "stop" feeling guilty about setting boundaries with your mother - because I have yet to complete the process. But I do have some advice for you that truly has helped me.
When we are just learning about narcissism in a parent and how it has affected us, you begin to see your feelings come about in two parts for each encounter with that N parent:
1) Discomfort/Anger that we are being hurt or affected by behavior of the narcissist, or discomfort/anger that we are compromising our own beliefs or feelings to make them happy
2) Guilt that we are feeling that discomfort or anger, because everything we have been taught by our N parent and society is to honor thy mother and father, no matter what the circumstances
Here's the good news: when you begin to realize these as two separate feelings, you can take action to eliminate #1! You begin to set boundaries. When you set boundaries, you stop the actions that are causing the emotions in #1. And I don't know about you, but having only one bad feeling is better than two ANY day.
Boundary setting is very difficult, but becomes easier with time and practice. When you set a boundary with your mother, you will feel initially liberated and you will begin to see the positive side: you are preventing yourself from being hurt.
The guilt, #2, is still there for me. However, I am confident that this has improved over time, and will lessen with time and experience. And the nature of the guilt has changed. It used to be I felt guilty about being angry with her because I felt like a bad person. Now, the guilt is because I feel sorry for her and don't want to hurt her (pity). Perhaps you can't see the difference, but I feel it. And now, when it comes - it is more recognizable, "oh, yes, here comes the expected guilt that comes with boundary setting, it will pass and I will be fine and so will mother".
My suggestion for you for the Christmas weekend is this. While I think your husband is smart in trying to protect you, I don't know if its a good idea to preface the visit with a statement like: "listen, mother, I don't want to hear any bad talk about Dad when you come this weekend". I think you might open yourself up for some defensive resistance and start off on the wrong foot - and she will enter the house feeling defensive.
Instead, I would go about the visit assuming she will not bash your Dad. Then, when the first situation arises and she makes a comment about him that makes you uncomfortable, address it immediately:
"Mother, what you just said makes me very uncomfortable. I would appreciate it if you would refrain from discussing your opinions about Dad during our visit this weekend" and then change the subject.
This will be less likely to evoke an argument - and she might be less likely to do it in the presence of your husband anyway. Additionally, the statement indicates that YOU are uncomfortable, not that SHE is misbehaving. This tactic has been very effective with my Nmother - because she thinks it's MY problem and not hers.
I imagine she may still react, if she is not accustomed to you setting boundaries. If she persists, I would suggest "I'm sorry you feel that way mother, but I've told you how I feel and need you to respect my request". or "While I respect your feelings, I really prefer not to discuss it any further during your visit".
You are correct in assuming she has no clue about her behavior. And you can count on her never understanding. The alternative then, is simply to take care of you and your feelings.
good luck. The holidays are difficult for us with dysfunction in our families.
CC