How can I best salvage my 240V GFCI spa disconnect wiring?
You CANNOT re-purpose a green wire smaller than #4 as a neutral, even if it is insulated. Is the #6 ground green all the way? 
If so running a new neutral in this case is your only option.

The ground wire in the outside portion of the wiring for a hot tub in conduit CANNOT be bare. It MUST be green insulated.

I would use a GFI in the main panel and a regular breaker in the spa disconnect. Easy-peasy.