Here's what it says on a Canadian legal site:
The mediator’s role is to help ex-spouses communicate, express their feelings, and resolve their conflicts, all in an atmosphere of respect and trust.
A mediator, however, is not a therapist, an arbitrator, or a lawyer and he does not represent either one of the parties. He cannot give you a legal opinion regarding your particular situation
A judge has the power to change your agreement or to send you to renegotiate certain clauses if he believes that the agreement is not in the best interests of your children. This can happen if, for example, you decide that there will be no child support paid although the parent who is supposed to pay can afford it.
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Sooo, it seems that the mediator can only help facilitate the discussion between the spouses, and that the agreement reached can be overturned by the judge.
I used to think that we were going to be reasonable about this and that XH was in agreement that shared custody isn't desirable for another year or more, but then XH turned around and tried to become Father of the Year and it willing to fight me tooth and nail for *his* daughter.
My lawyer has told me that if mediation degenerates and my XH become verbally violent, she can write me a dispensation due to "cruely" and I won't have to continue with it to obtain my decree of divorce.