Divorce is tough on me and kids. Think things through.
*** Not a lawyer ***
The general idea is that the child support is for the benefit of the child and can’t just be waived by the parents (despite the fact that there’s no requirement the receiving parent actually use it for the child’s benefit). On the other hand, if the agreement you refer to is specifically addressing child support, and is part of the divorce decree or is part of a valid (notarized) separation agreement, then I believe the usual presumption is that that agreement stands with regard to retroactive support (meaning “before the start of the court case to determine child support”).
You should probably hire a lawyer to discuss the specifics of your case, especially with CSE involved. While this forum that Rosen Law Firm provides is great, they can’t get into the specifics of anyone’s situation for a number of reasons.
If your eldest daughter were still in high school (college doesn’t count) and under age 20, then the NC Child Support Guidelines would basically calculate the support you would be paying for the younger daughter and the support your ex would be paying to you for the older daughter, and whichever of you would “owe” more would pay the difference. See Worksheet C for the details.
Now that the eldest daughter is age 20 (or as of when she graduated high school, or dropped out, or “fail[ed] to make satisfactory academic progress towards graduation”), it doesn’t matter at all how much you support her as far as the NC Child Support Guidelines go.
Again, though, a specific court order (e.g. your divorce decree) or a duly-notarized separation agreement could override all that, at least until a new court order (or valid modification to the non-incorporated agreement, if possible) changes things.
That doesn’t matter as far as child support goes. If anything it might hurt your case by making it seem like you didn’t care enough to remain involved.
I’d think you could enforce it, although it might take court intervention to force the ex to allow it if she won’t cooperate. And if the daughter doesn’t want to for whatever reason, I have no idea whether it would be better in the end to force her anyway or to respect her wishes and hope she decides to change her mind.
I’m guessing the amount of visitation wouldn’t be at least 123 overnights per year, so probably not. Worksheet A doesn’t take into account the time that the child spends with the non-custodial parent. If you would have her at least 123 overnights per year then it might, because then you’d be on Worksheet B.
A party can seek retroactive support, but it sounds like you may be able to argue that the child support agreement you entered would bar her claim for retroactive support.
If you want visitation, you should consider filing a claim for custody or a motion to modify. We have a good article on our website about when a child gets to decide where he lives or if he visits you should review.