[[WMG: The spirit of the cauldron was what turned Luna into nightmare moon ]]
It seems that it had great power beyond what it utilized, perhaps it corrupted Luna...
** Keep in mind that these are two non-related universes, with no common points. There is also the temporal issue, in that anything after the film would be using a useless cauldron whose dark magic was nullified while anything before the film began takes place in the film's presented back-story. Additionally, Luna's conversion into Nightmare Moon was perfectly explained in that she wanted to be respected and to achieve this gave into using her powers through her own darkness thinking that as the new ruler she would have the respect heaped upon Celestia. King Sombra might be less-absurd but then you run into the problems of combining the two non-overlapping universes.

[[WMG: Eilonwy is not actually a princess in the Disney movie.]]
While she does introduce herself as "Princess Eilonwy", the Horned King actually refers to her as a scullery maid. While you could maybe argue he called her that because he kidnapped her and turned her into one, he calls Taran a pig keeper immediately before that. So it's possible Eilonwy actually ''is'' a scullery maid in the Disney film, possibly working at the palace, explaining her having a nicer-looking dress than, say, Cinderella's attire as a servant, but not princess nice (even compared to Snow White, whose dress is the most similar). Her dress is neat because she's just a palace servant, one that isn't supposed to be mistreated, but she's still just a kitchen maid.