Check out the Brac Reef Beach Resort on Cayman Brac... I feel confident that the hotel management will be willing to work with you to make sure that you and your wife have a great trip because they host Dive Pirates vacations every year for disabled divers. Good luck!
Little Cayman Beach Resort or Cayman Brac Beach Resort(both owned by the same people & operated the exact same way)...........I think they are both set up for you, I'd give 'em a call
One more thing, Cayman Air flies a variety of aircraft between Grand Cayman and Brac and Little Cayman - including a daily jet service - so that might make it a bit easier when traveling with a wheelchair as opposed to one of the smaller propeller planes.
If you want to include more tourist-type activities, adding on a few days on Grand Cayman can be fun.