If your departure time is 3:50, your DME p/u is going to be between 12:30-1:00. You may get a time of 4:10, but it's better to err on the side of earlier. So, figuring you have a p/u time of 12:45, you need to be back at BW to get your carryon bags out of storage no later than 12:20ish..it could take them 10 mins to do this. You should be at the DME bus stop at least 10-15 mins before the stated p/u time...this time is the time the bus is supposed to leave the resort, not the time you should amble on over to the stop!!!
So, if you need to be at BW by 12:15, you will have to be walking out of DHS by 11:50ish....it takes under 20 mins to walk. If there is a boat there, loading, then take the boat and get off at the S/D and walk over to BW. If there is no boat coming in, or in and loading, walk back.
This means you have to be done eating at Sci-Fi by 11:40. Doable, but way too many variables to count on.
I'm not really sure that taking a cab is going to make a huge difference. I guess it could save you 10 mins. But I have to tell you....if you're worried about saving a measly 10 mins, I wouldn't be keeping that ADR!!! Not a good way to end your vacation...worrying about missing your ride back to MCO.