Paying an attorney to tell you what you could expect if you went to court is money well spent.
In my case, I pay child support but also share 50/50 costs for childcare during working hours, medical, activities etc.
I would agree with the advice that you focus first on making sure the kids are safe and you are in control - you can always make exceptions should he pursue recovery. Most mediators handle the JPA first so that the parenting plan is not being impacted by financial negotiations. In your case, that would be a wise approach.
Above all, make sure the mediator knows that your primary concern is the kids and their well being. Be factual, honest, and measured, not emotional or accusatory, in the way you share your concerns. Part of what the mediator will be evaluating, even if not consciously, is how you would fare in court. Be respectful with him or her as if you were dealing with a judge.