I think I figured it out. Noticed problem was fixed when I removed the Unoffical Skyrim Patch – I’m guessing it runs the scene that Bethesda scrapped that causes the spouse and wedding guests to sandbox after the ceremony.
Of course, if the scene isn’t stopped the “Love Interest” and all the guests will never move, they’ll just sandbox forever, so I added a manual
RelationshipMarriagePostWeddingScene.Stop()
to their “Where do you want to move dialogue” which will immediately stop the Sandboxing.
This will only work for modded followers, vanilla may still be bound to the bug, but I think it’s a USKP bug and unrelated to this mod (or a conflict caused by editing the same quest).