Vik, she is running over you. I'm glad you see it. You're right to ask for help and try to step away from being in the middle.
I don't know that I have the answer. You could tell your W the same thing you said to your MIL: "I do not wish to be in the middle of your relationship with your mother. It is not fair that you ask me to lie for you and I will not do so. I also am not intending to discuss your business with her - that is between the two of you. This is my stance with both of you. I will not be your go-between".
And you can say that to either person. Just flat-out refuse to discuss ANYTHING with EITHER of them. Find some current events to discuss, or the weather. At this point discussing politics or religion is probably safer.
