I agree about Epcot being your longest - with World Showcase, it seems to take time to enjoy each country.
I would then say do MGM, so you make sure you are able to do what you would like. Even on busy days, we are usually able to do most of what we want around mid afternoon - but we do get there at opening.
Downtown Disney has lots to see, but not sure a whole day thing - I am able to be down there for a few hours and then I head to West Side for a bit.
Since Pleasure Island is only opening in the evening, any evening you have free (and have the energy) I would go.
Also, since you are staying at the Swan, seeing the Boardwalk will be fairly easy - it won't take near as long as the others.