Just sharing how H is troubleshooting the CS transition when SD17 turns 18.
She turns 18 on the same day of the month that H usually pays CS. And, in our US state, CS must be paid for adult children up through age ~21 if the child is attending college & not failing out.
To solve the first issue, H plans to pay Mom the relevant fraction of SD17's CS (i.e., if he usually pays on day X, then Mom will receive X/30 of that amount), with the remainder going to SD17. After that month, Mom will only receive CS for SD15, with SD17's amount going to her.
H is working directly with SD17 to set up direct deposit to her bank account, and that seems to be going okay.
This is likely a better plan than trying to work with Mom to say "well SD17 is born in this month, so SD17 will receive the full amount for the month".
As far as I know, H has not talked with Mom about this. I don't really anticipate gratitude from her, and I think she'll probably complain to the kids, but at this point SD17 is pretty determined to move out of Mom's house so I don't think Mom will be able to guilt her into staying for financial reasons -- but we could be surprised
(Despite every other ongoing conflict we've had, weirdly Mom has been okay with H paying CS by check directly to her. I think H sold it as "otherwise you get less because the CS agency takes a cut")
As much as I wish that we had a two-family setup where the parents could talk with and cooperate with each other, we don't, and so that means that the kids have had to grow up a bit more and work directly with H on a lot of things that a cooperative family would've handled between the parents.
Anyway, hope that problemsolving is helpful to others out there.