Where exactly on Koribban is Dustil Onasi?

Answer 1

He is in the Sith Academy; you must have had Carth with you while landing on another planet (you must have persuaded him to talk about his wife and son, then someone will come up to you when you exit with Carth and ask him about his son). Bring Carth into the Academy with you, his son is in the only "student's room" without an occupant. The... bottom right room in the western wing, I believe. Or the bottom left. Either way, it's the room at the end of the hallway, whichever hallway it's in. But he's definitely in the western wing.
If he's not there, you may have missed it. You don't *have* to find Dustil to beat the game though....