The constant bait and switch with character deaths is really starting to grind my gears, but I don't think they couldn't permanently kill off any of these characters, if they wanted to.

The time bubble applies to more than just whether a character is alive or dead. The Nanamo in the CUL quest acts completely different from the Nanamo we know in 4.X. I don't think breaking immersion in quests like this would keep them from killing off a character, since that immersion already gets broken depending on the order you do the quests in.

Urianger being a permanent NPC quest giver for several side quests is some pretty good plot armor. Theoretically they could add in a new NPC to take his place if he's dead in your game, or maybe have Urianger's Notes where he used to be standing or something, but that would take some effort, whereas all these other NPCs appearing in cutscenes can be handwaved away with the time bubble, with no additional work required by the devs if they did decide to kill them off.

I don't think these characters won't die because it'll cause issues with sidequests, they won't die because the devs don't want to kill off popular characters.