You can remove Potassium Sorbate the hard way.
It was describes in Jack-Kellers web-log.
Make a strong yeast starter made from bread yeast.
Add that to the cider.
Let it ferment until it stops.
The bread yeast will by then have consumed the
Now rack the wine, and add another yeast starter.
This one will have an environment without sorbate
and can ferment to the end.
Read the full story here:
and look for the April 24th, 2007 entry
It might help in your particular case.
Try with a small batch of about a gallon and go from there.