Where did the alternate endings come from?
They were, for the most part, suggested by passages in the original story. George MacDonald presented several characters’ imaginings about how things might turn out if such-and-such didn’t happen, which makes it very easy to interpret these as alternate endings. Some of them were pretty silly things that he obviously didn’t mean to be taken seriously, but as this is a fantasy / fairy-tale world, we can go ahead and do them anyway.