There is no bus service from any resort to another resort. To get to the Boardwalk area you can walk or boat from either the Disney Studios or the International Gateway of Epcot, which is near the United Kingdom. The walk from Epcot is maybe 5-7 minutes tops. From the Studios more like 15 minutes.
You are correct that you would need to go to DTD and take a bus back to your resort from there. I believe the service to DTD runs pretty late...like 1am or so.
The easiest solution is to take a taxi back to your resort. Just enter any of the Epcot resorts (Boardwalk, Yacht Club, Beach Club, Swan or Dolphin) and walk through the lobby and out the main entrance. Taxis are normally waiting outside, but if one is not there a Cast Member (at the Disney resorts anyway) will get one for you right away. We caught a taxi outside the Boardwalk resort back in June with no wait as several were outside.