I'm so sorry. I know how nerve-wracking it is. I don't feel I can give any advice, because I failed miserably in mediation. My biggest mistake was interrupting X. I got put out in time out, essentially, while X worked his narc magic on the mediator. So I guess that's all I have to offer you- DO NOT interrupt. I knew this, and I never thought I would have, being as there was so much at stake. But it's very hard not to correct blatant lies.
Also, don't cry. I knew this, too, but failed to hold in the tears. I guess the lesson is;
It's all about self-control.
Best of luck.