I've never done a combo of the rental car/limo service, but I *have* done two 1-day rentals.
The last time I stayed on-property was before DME started. Was able to get cheap rentals through Alamo that were comparable or cheaper than getting three people to/from the airport on Mears.
So, we rented the car at the airport, and then the next morning we dropped the car off at the Dolphin and ate at the buffeteria there before heading over to Epcot.
We reversed the process for our departure, picking up the car the afternoon before we left.
Oh, and the drop-off at the Dolphin is very simple. The valet will accept the car return, and if you used a credit card for the rental and don't need to change anything about it, you don't need to stop at the rental desk.
-Rob