(re: re: King’s Quest 6, and illogical puzzles… (mini-spoiler))
err… no, by the time you get to the Land of the Dead, you’re already on the path for the “fuller” ending.
I think the branching happens depending on how you use Beauty’s dress. You can put it on and get into the castle pretending to be a maid. That leads to the “short” ending. Or you can get caught by the druids and use the dress to put out the fire when they trap you — which cuts off your access to the castle wearing the dress (since it burns up). So at that point you’re “stuck” on the long ending (and do need to play the bones once you get to the Isle of the Dead).
-emily