If you have a save before talking to the alchemist and giving him the 3 ingredients, you can reload and do the other quests first.
It is fairly clear that this is a main plot quest (the chieftain's son says he will tell you how to get to the Raanaar temple if you find a cure for the plague). However, there is not much warning that the plot will turn linear. He does ask if you are ready to go or not, which indicates something's up, though.
