You can take the bus to MK and then the bus to WL. The bus ride to WL is approximately five minutes long The boat ride is great, too, and I would definitely do this if you found yourself with a little extra time.
If you take the bus to Epcot then you have to walk through the entire park to get to the YC. If you take the bus to HS then you can either walk or take the boat to YC. It will be quicker to go to HS instead of Epcot.
I agree to take a taxi. Sometimes the buses are shared among all 5 Epcot resorts, which can make for a long time getting there (especially if YC is the first or second picked up and has to makes stops at 2 or 3 other resorts before heading to MK). Good luck.