Just read your post and had to comment for what it's worth. I'm 6 years into this and my H was one of the nuttiest, nastiest men MLC guys around.
But this I learned - good deeds work better for you in the long run. Despite the fact she witheld the kids from you at Thanksgiving, I honestly feel you should go the extra mile for Christmas, be the bigger person and let the kids spend some time with her - even if it is a short amount of time. It REALLY is in the kids' best interest to do so.
Visitation isn't about "getting even" or "sticking totally to the agreement" - its about being flexible. Maybe, just maybe she will lose some of her iciness if you make a kind gesture.
I honestly feel you should consider this or I wouldn't have bothered to post.