Are characters who appear in multiple separate questlines immune to being killed?

[Nama]

Well given enough work no one has 100% plot armor. Safest a npc can get is being a static npc that gives quests ie Urianger. Anyone else really comes down to if SE is willing to go back and add new flags and changes to quests. Given how much devs have to work on and the pace they go they would likely avoid going back and tinkering with "done" content. Any inconsistencies could be written off as echo or timeline "bubble".

Npcs fall into a few tier examples based on how much work it would take to change/remove them.
Extremely unlikely: System reasons, gives quests etc -Urianger. Too popular/represents XIV - Y'stola

Very unlikely: "plot armor" Minfilla, appears in numerous sidequests - Godbert.

Unlikely: appears in single sidequest or referenced by often by others - Nanamo, GC leaders.

Somewhat unlikely: only appears in MSQ, never mentioned by name outside of msq, is a instanced npc - Yda. Papalymo

Likely: recently introduced npcs - Moenbryda

Extremely likely: no name npcs, wears a red shirt...