I'm by no means perfect. However, I remember when I was in Sunday School as a kid, that my teacher told us a story about a person who had many reasons to hate another person after what happened between the two of them. The teacher pulled out a backpack and started an object lesson by putting a rock in the backpack for every time the angry person thought of the bad things done by the other person as well as one for each day that passed without forgiving the other. In the end, the backpack was full of rocks and the lesson is that it is such an enormous burden to hold onto the grudge for the person who was wronged. As an adult, I learned that holding grudges can adversely affect a person's health.
I think it's important to remember that we don't have to like the person, we don't have to like what they did, and (if still living) we don't have to trust that person again. Forgiving them is more for us than it is for the person we are forgiving. It helps us to live our lives more deeply, with better health, and enjoy life to the best.
I would be very bitter and upset if someone did what your MIL did you the two of you. However, for the sake of my own life and the lives of the people around me, I think I'd work very hard to forgive her so I could be at peace instead of in pieces. I think it will help that the debt she incurred is paid off, if I read the post accurately, so that the financial obligation isn't hanging over your head any more. It's so sad that the last payment was when your Dear Husband died and that he never got to enjoy the benefits of it being paid off the way you probably have.
My advice, forgive and make a list of all the wonderful things that have happened in your life including the uncle that found your husband so you could enjoy him while he lived. Find peace and happiness, you deserve to be happy!