Either way, she probably won't listen to me anyways. Not that I blame her, I wouldn't listen to her if roles were reversed. I'll probably just end up filing a complaint for modification a few months before my youngest moves up and let the judge sort it out (?).
Pardon me for seeming obtuse, but I've read and re-read your posts and still uncertain of what is that you wish to have modified.
You've mentioned that the "side agreement" fixed child support less than what would be called for by the guidelines. Which would seem to suggest that there is an existing court order based upon the guidelines, but you haven't directly alluded to any such court order.
However, I'm going to assume that such a court order does exist - otherwise why the need for the side agreement?
And if that is the case' I see no reason whatsoever that if it is your desire to move for modification based upon a material change of circumstances that the "side agreement" would be a significant or complicating factor.
The reason is that if mom successfully argues that the side agreement be enforced, it would serve to modify the court's order, which you will be asking to be amended again.
But if you think that you are going to just throw all of this before the court and let the
"judge sort it out", you'd better do some rethinking.
Attorneys that follow that trial philosophy soon find themselves looking for an honest occupation.