' don't wanna get in a battle of semantics with you, drC
But I never said the existing order would be vacated. By its terms (as relayed by OP) the WA court order will expire upon certain contingencies or by operation of law; i.e., the child marries, dies, joins the armed forces, or graduates from HS/attains age 18, etc. Thus there is no need to "vacate."
While I'm not licensed in WA, I've handled a number of CS cases across jurisdictional lines. "Alternate parent" may be a term used in the nether regions of Joisey, but it is not one that is universal to any other jurisdiction with which I am familiar. Most speak in terms of "custodial" vs "non-custodial" parents. In the present case neither parent is "custodial" therefor both may (or may not) be liable for ongoing support until the child reaches age 18/graduates from HS. Admiitedly it is a fact dependent scenario (as to emancipation, etc) and the current order stands until modified (vacated, if you wish) by a court of competent jurisdiction.
That being said there are only 5 months until the question becomes moot...OP will have to decide if its worth the hassle/expense to hail his ex (and the child) into court to deal with his concerns.