Is there that much in the deduction to make it worth her while, or is this just the only thing she could complain about?
Regarding that Motion to Dismiss, you might want to buy an hour of a lawyer's time to help you prepare it, but otherwise file it and argue it pro se. This is called Unbundled Services with lawyers, and they don't represent you, but help you get the paperwork correct. You don't argue your case in the motion, but essentially you will be arguing that she agreed to this, that she is required to mediate, and that her motion is premature. Your One-hour lawyer should help you get that arguement together.
It's up to you if this is the opportunity to show her you will fight future modifications tooth and nail. I wouldn't make it easy on her, in fact, any counter motion should put some skin in the game for her. You can't ask for fees when you are pro se, but if this escalates - you be demanding fees for your bulldog lawyer sitting next to you, growling and snarling at the mother.
How the kids?
Volenti non fit injuria