From Epcot, you could also take:
1. bus to Fort Wilderness (and then internal bus to Pioneer Hall) or
2. bus to Wilderness Lodge , and boat to Fort Wilderness...this would be my choice if I had no reason to go to MK first.
To get back to the Boardwalk, it depends on which show time. For the late show (9:30), they have busses that run just from the show to resorts. When you come out of Pioneer Hall, go right and you will see some busses lined up. They group them as makes sense to them. Just ask a bus driver which one they want you on. This can be true of the 7:15 show too.
For the 5pm, and sometimes 7:15, you have to rely on the boat. One boat goes back to MK and you can grab a bus to the Boardwalk -- again, unless it is late and the park/transportation is closed, that's when you would use the special bus. You can also boat back to Wilderness Lodge or Contemporary, or use Fort Wilderness bus system -- but that will not get you directly to the Boardwalk.