If it helps, I made a similar mistake, using cider that had potassium sorbate in it to make a 5-gallon batch of Graff. After reading several older threads I decided to pitch more yeast (twice) and then wait. The results ended up being better than I expected. The graff definitely fermented out -- it's even a little dryer than I wanted.
There are some flavors that are not quite what I expected from it, but this is my first attempt at graff so I am not sure. It's definitely quite drinkable -- I'm going to open one now, if fact.
Bottom line, I would say let it ride