The part with Migo isn't correct. He won't attack you, if you have the Yarrow flower in your inventory, speak with him, then offer him the flower and he'll give you his ring. What you do with it, is up to you, but to advance his quest, you should give it to his daughter, Magister Yarrow (his little Yarrow girl). The flower can be found pretty easilly, so there's really no point in him attacking you. For solving this quest, Yarrow will give you the key to Fort Joy's main gate (where Magister Boris stands).