You only have two full days and in addition to Amalfi/Positano you're considering Ischia? Given all that, Sorrento makes a lot of sense. It's not as idyllic as Amalfi but you could take the ferry from there to Amalfi or Ischia for day trips.

Can you see if the hotel in Sorrento could keep your car for Thursday night (might even make sense to pay for a room for Thursday night that you don't use). Spend Wed night in Sorrento, take the ferry Thursday AM to Amalfi spend the day/night & Friday there, take a late ferry back to Sorrento and spend the Friday night there before a Sat departure? Alternatively, if you want to go to Ischia, take a Friday AM ferry from Amalfi to Ischia. If you didn't have the car, another option would be to spend Friday night there and then take a ferry from Ischia to Naples Sat AM depending on flight time.