My biggest concern is what I should discuss with the counselor when she is present.
As little as possible. My personal experience and opinion is MC can sometimes turn into divorce preparation counseling. Unless you have a pro marriage counselor, it usually seems to work out this way.
To go to counseling both parties have to be all in, in their desire to save the marriage.
Is this the case with you? Or are you hoping the MC will some how save your marriage?