root when i told her paper with $X wasn't set in stone, she threatened to file a complaint with domestics. what do you say there ?
Just tell her you need receipts for your legal records. Tell her this is to help protect her as well (make everything sound like you are doing her a favor). This is important even if you have a just have a "mediator." Suppose things escalate and she wants something very unreasonable. You need to be prepared either way and having receipts is a necessity.
What is a complaint with "dometics?" Unless there is a legal order with a specific amount of money, and something legally telling you to pay it, you don't need to give her anything. As a courtesy, and to "be nice," you can give her a reasonable amount of money to help with some bills and things for the child. Never pay more than what's necessary.
As far as a reply to any threats just tell her you don't have any extra money and are doing the best you can. I personally think the less money you squeeze out to her the better. Don't make her too financially comfortable with this situation. But talk nice even with the threats... just say you are sorry, but you just don't have the extra money (even if you do). Plead poverty and be sorry about that. Don't tell her it's because of her wanting this D. Never never never blame her or point out the obvious like some annoying parent figure. She'll figure it out on her own and that's much more effective then the preachy (using my nasal voice here) "I told you so."
There is no arriving, ever. It is all a continual becoming.