I finally cleared the mission! Did you ever figure it out?
If not, I can help: After losing the iron key, you go north and teleport. If you are teleported to the microdragons, then make your way to question 3 (South, East, South).
Choosing the last option, you will be teleported to one of the granitedragons to the east of the question. If you were teleported to the old granitedragon, head North then West.
Otherwise, head North until you fight the centaur boss. Then go West, and then North until it is no longer possible. Then head West until you recover the iron key.