That's an interesting question; I hadn't through about riding the train from MS to NOS. If the train is running, I would probably consider doing that. I don't think it'll save you much time, but it's probably more relaxing.
Otherwise, yes, you can probably walk up via the Emporium, but you'll still encounter the crowds when you have to exit before Carnations. You could try to slip back into the stores (Gibson Girl, Candy Palace and Casey's Corner) around Carnations or Gibsons Girl, but my guess is that you'll have a hard time merging into the traffic at the end of the stores. The walkway skirting along Jolly Holiday tends to be filled with guests heading out of Adventureland, so you'll have to get across them to merge with the crowds on Main Street heading to Frontierland.