You're being as negative as she is. Quit going down negative spirals about your MIL. She only bothers you because you let her, much the same way my mother used to get to me.
Think of the conflict my mother caused between me and H. It isn't worth it. You can choose how you deal with her. How your W responds to her is her problem.
I think you were right about S5.
Re your W's moodiness - she IS moody because a R didn't work out, her R with you.
She has just got a D - she is bound to be depressed, esp. if she thinks she has made a mistake and is too proud to admit it.
She is going to be cranky and hurt and frustrated and be grieving the lost M even if she was the one who walked away.
Just hang in there and try to let her grieve without taking it too personally.
Most of the negative stuff they say is just BS anyway.