Just catching up - agree with others about the mediator who seems to change based on the wind...
Although I have done a bit of mediation (for CPD mostly), not sure I agree with her meeting you and DH separately. Initially, yes of course, to be able to get the history from both sides, but she has crossed several lines and appears to be jockeying for position, i.e., mentioning that she knew your hot shit lawyer. She did not need to say this, and either she cannot filter, or she is trying to impress with her pedigree. I cringe when she keeps changing her view based on who is in front of her. I also really question her furthering the narrative that you haven't adapted as the reason why it's all going so wrong, such as highlighting her own qualifications in change as if to say that she is an, 'expert,' and gets it, when in fact the issue seems to be a toxic family who are hostile to the newcomer casting light on their dark and dirty secrets.
Is there any way you could try and get through to your DH - maybe a last ditch effort on neutral territory? He also has been lied to and whether he chooses to believe it or not, his mother has manipulated him. It's not easy to admit you have been duped, especially by his own mother.