I'm not aware of any rental companies that service both WDW and Port Canaveral. However, if a WDW vendor also services the MCO area, you may be able to make arrangements to meet for return at the airport. Some WDW area vendors and some feedback are listed in the disABILITIES FAQ sticky near the top of this forum. I believe Brevard Medical serves the port. You might need to contact a few to see who will work with you.
As for transport from WDW to PC, the DCL transfers *can* be accessible -- you'd need to note it on the reservation and I recommend following up closer to the cruises as well. A private car service *may* be able to transport if the ECV breaks down for travel, and if there isn't a ton of luggage too.