It's going to take you at least an hour to get out of the airport, so 3.30 should be about right.
On the way home, you can check in at Downtown Disney early (8-1 ish I think) so you don't need to get to the airport til quite close to your flight. I'd leave at least an hour to get through all the security checks just in case, so a drop off at about 5 should be OK.
It's true you'll pay for an extra day, but I should think it's worth it. We had to get our car back by 3.30 as we'd booked it before DTD check in re-opened, but if we'd had just that bit longer we'd have had time to go to a park for that last little bit of magic!