Like has been said plenty of times. The children agreement is key. And sticking to it. However, if she gives you one of her days and you are free, I would take that as a bonus and have the children over. Document every instance. Also, she may ask to trade days, you don't have to if you don't want. Just say you already have plans with the kids or on the off day. Document everything for possible need in a crazy custody battle.
I hear what you say about the MIL. My MIL, SIL, and BIL were the same way. It wasn't until I ended their communication did my W see the poison they were filling her head with.
Me:49 W:45 M:19 T:22 EA confirmed and ended 8/2014 S:19,17 D:9,5