There are no clear winners in this matter. Both parents appear to have inherrent problems regarding parenting this child. The court will award custody to the parent who is best able to make the child available to the non0custodial parent but your previous DVRO is problematic. I suggest that you spend the cash for an attorney on this.
The advice that you should not be disparaging make sense. However, that does not mean that you cannot talk about the facts. I would recommend that you talk about the progress that you have made personally, and about your desire to be a good parent for your child. You should also talk about your relationship with your ex's parents and about how they are supporting you. Come with contact information for them; perhaps you can bring a letter from her parents. You can talk about the things that she has done that concern you, including her failure to allow court permitted visits, etc. Just don't use pejorative language to describe her. If the person she is now living with has a history that suggests potential harm to your child, you should bring that up as well. Most importantly you need to be thinking about the best interest of your child, and talking to the mediator about the best interest of your child. If your comments suggest that you're more interested in talking about the shortcomings of your ex, then about what your child needs and what you can offer, then you will not make a good impression.