I slightly disagree on your analysis about the plot: if you consider the story as a coming-of-age narration, you may find out that actually Merlin is preparing Arthur to became a king all the time! This detail is pointed out during the movie in three distinct moments, i.e. in the introduction, when Arthur falls down in Merlin's house and, obviously and when Arthur becomes king, after extracting the sword from the stone.
Merlin, as you said, is showing to the king-to-be Arthur that the world is not only darkness and it is not ruled only by mean of violence: Merlin teaches to Arthur (and to the spectator) that a wise ruler have to consider different point of view and to know the wonders of the world.