Merry Christmas and Happy New Year! It sounds as if you have lots of good going on in your life, but until you get the D done, stay cool.
Your X showing up at the hockey game was to find out things. You see, she had 2 things before: her married BF and you. You loved her regardless of anything. But now there's somebody else and this so sounds like jealousy to me.
I agree with wii. Do not bite. As hard as it may be not to. But do think in terms of what is best for the kids.
None of this sounds good to me. When so many people are involved kids tend to feel lost. Your daughters need something stable. So you give that to them.
One thing that struck a chord with me is the fact that your W has the mediator's ear. Be careful there. She works for an L and I would hate to think she has a leg up. Either way, do what's right for your kids and you will be fine. No matter what happens.