“Do what you feel in your heart to be right, for you'll be criticized anyway."
You're going to need to follow what you think is right and shut out MIL or anyone else who thwarts those efforts right now (kindly). I qualify that when I say "what you think is right," though. Obviously, we need to stick with the techniques. For me they don't always "feel" right but I know that they are because they do get results. There are moments I want to tell my H exactly what I think of him at the moment, but it isn't right.
I'm so sorry that you are going through this. I actually do think that it is a good idea that you are there to support your children, but are you preparing yourself? Meaning, have you walked through the scripts in your head for the encounter with W and OM? This is going to be a big test. You are doing so great so I would make sure that I was prepared for this.
Please do let us know how you are doing. I wish there were magic answers that would make this go away, but unfortunately it is stuff that we are just going to have to go through it. There ARE better days ahead regardless of the outcome. I know that's hard to believe when you are in the thick of it. Heck, I'm not so sure I believe it sometimes, but there are better days. Hang in there and let us know how it went.