So sorry to hear etc. I am dealing with the same thing right now. My H wants to work on the separation on our own however I have contacted a lawyer, as I feel that is in my own best interest and I am worried about everything being split fairly and having enough money to support my family. I do not know the answer to your question of using a mediator vs using a lawyer. I guess it is your own personal preference. With a mediator I imagine you would be sitting down and negotiating with your W a lot more. If you would rather handle things on your own without much discussion with your W, than perhaps a lawyer is more suited for you. What are your main concerns? Do you feel you need a lawyer to protect your own interests?
I am not sure at this point that you can persuade her against D as she is already in contact with a mediator. For sure I would let her know that D is not what you want but that you are willing to work with her if it's what she wants. I am in Canada, not sure where you are located, but here the D isn't finalized until at least one year after the separation, so there is still time that things could change in the meantime.
Take care, -cp
M: 8 yrs T:14 Twins:7 S:5 BD:'NLILWY': Feb/2013 Mar/Apr/May: MC June: "living in limbo" Sept 12: H moves out Oct 20: reconciling Jan-Feb 2014:MC Feb 2014: separating, and H moved out.