As you're walking along the pathway, you'll pass the BWI tennis courts. There is an adjacent service road that runs parallel to the walking path. There are steps that lead to the service road and as you see the BW pool, cut over and walk around the pool and out into the courtyard area where the Wyland Gallery is located. From there, continue on around Flying Fish and the rest of the Boardwalk area to BC/YC.
I'm not really sure if that shortens the walk for you. You won't be backtracking through S/D but you'll be walking around the lake.
What will get you out faster at Fantasmic is to wait a couple of minutes after Fantasmic ends and then walk across the very front of the amphitheater to exit up the last set of stairs - the last section of Standby. You won't be stuck in the huge crowd trying to exit out the main exit/concession area.