The above mentioned quest is impossible to complete for anyone above level 50. This means that if you have leveled a character to max you cannot go back and do the Legion Alchemy quest chain.
Specifically the part of the quest that is broken is the Silver Hair of Khadgar portion. You must use a Skaggldrynk potion to become invisible to get his hair, class stealth abilities will not work. Problem is the potion cannot be used above level 50.
Even while party syncing with a lower level character, the lowest you can get is level 61 so still not able to use the potion.
EDIT: Sooo in the process of trying to do this, I went to the quest marked area and attempted to use the potion. Even though the tooltip says it wont work, and it doesn’t work in other locations in game (I tried) it DOES work if you are in the marked area for Khadgar. So yay, at least it is able to be completed.