Removing the underbelly in just the area were the duct runs is really not that bad at all. Best and probably only real fix is to just open it up, pull the flex aluminum duct out and replace. Once you get into it you'll realize it is a lot easier then you are thinking. Go into it taking your time and you'll do fine. If I had to guess, it's probably a 2-3 hr job start to finish taking your time.
If you don't remove the duct, the butter residue no matter how you try to clean it will attract insects, critters and also will mold.
