DME, I assume that's the shuttle you're talking about, picks you up 3 hrs prior to flight time. It gets you to the airport about 2.5 hrs prior to your flight. Even if you drive, you should still get to MCO 2 hrs prior....so you're gaining 30-45 mins. I guess that if you rented a car. You could drive to the park, with your carry on bags in the truck, and then drive to the airport right from the park.
So, if your flight is at 7:15pm, DME will pick you up around 4:15. So if you're at a park, you need to leave there by 3:15 in order to get back to the resort to get the bus. If you rent a car, you would have to leave the park by 4:30ish. That would get you back to MCO by 5:00 to return the car. You should be at the terminal around 5:15ish, depending on how long it takes to return the car.
Doing to that way will give you maybe an addtl hour in the park. But remember, that's rush hour...you could run into a lot of traffic.