I think they could change Urianger's design and explore his story, even with the quest-giver NPC in the Waking Sands. In terms of timeline, it's pretty much accepted that the Binding Coils happened before Alisaie came back in Heavensward. There's also cases like Estinien not recognizing you in the Dragoon Job Quests after you've finished Heavensward's story. So I think that, if Urianger changed in Shadowbringers (or, let's say, even died), he could still appear in the Waking Sands with the quests. Most people would probably assume that this an event that is placed chronologically in ARR (it's a matter of looking at the quest level).

As it stands, right now in the MSQ, Urianger is in a coma... but he still appears in the Waking Sands giving me quests. So the timeline is already broken anyway lol