I recently completed Arah story with a friend who also needed the mission award and some guildmates. When we went to Fort Trinity after, I joined her instance, and was unable to talk to anyone or see any of the culminations; fair enough, I’d just not accept the credit for completing that segment, and then start it myself, like the game had allowed us to do several times prior.
Nope! On exiting the mission, I was immediately forced to accept credit for completing Victory or Death, despite not getting to actually experience any of the ending at all.