After making a new character to play through the Fangs of Asterkarn story, I went to one of my other characters, wanting to access Kurnhold and apply modifiers to my potions. This character has already unlocked Ultimate Difficulty, and thus cannot unlock waypoints on lower difficulties by consuming a Savior’s Merit. To this end, I asked one of my friends to make a riftgate for me in Kurnhold, at which point I was able to speak to the relevant NPC and unlock the Potion Cauldron on my old character. All fine and good, but for some reason, returning to singleplayer has re-locked my access to the cauldron and returned the Wonderous Elixirs quest to my quest journal, prompting me to talk to the NPC again to complete the quest.
Advancing through the dialogue in a manner that should complete the quest does not do so. I appear to be unable to modify my potions any further on this character.