Universal can use backstage areas at their discretion, regardless of IP-holder concerns or not. The only things IP holders can prevent are an unapproved modification of trademarked visuals in the park or an unapproved usage of characters in an alternate environment. Universal could put a house right behind Gringott’s that queues through Ollevander’s and, as long as it did not altar any visuals in Diagon or use HP characters, WB/Jo wouldn’t have much say at all.
It doesn’t really matter what goes into KidZone; Apart from losing B79, it won’t really modify HHN much. Even Seuss Landing, which is the only known spot that was not authorized for HHN, queued multiple houses.