After completion, no. But you said you couldn't complete it. If it's not completed, then most quests can be abandoned and started over. (MSQ cannot be abandoned, though. I'm not sure if there are any others that can't.)

But what item? And what do you mean by the "correct" area of the bag? If you need to use or turn in an item, you should be able to do that from whatever part of the bag the system used for it. Or do you not have the needed item at all?