Why did King William give Derek that incredibly vague and unhelpful "It's not what it seems!" explanation rather than just saying "It was Rothbart! Rothbart transformed into a monster and took Odette!"? Because there's a good possibility he didn't even know it was Rothbart. Think back to that scene: it's night time, it's raining, Rothbart is standing a few hundred yards away so William and his guards can only see his silhouette and it's been two decades (or near to it) since William had banished Rothbart. Plus William only sees Rothbart for a few seconds before Rothbart transforms into the Great Animal and he gets attacked by him. Their carriage stopped, William got out and saw a threatening figure blocking them, went back to warn Odette, then it was all over in a few moments. Maybe he just didn't recognize his old enemy? "Out of sight, out of mind" and all that. Rothbart can't be the only black magic user to ever live and kings like William probably had to deal with threats like him to their kingdoms quite often (hence why the Forbidden Arts are forbidden). So poor William gave the best advice he could while he was in shock and dying. At least it did end up being kind of helpful in the end, even more so when Derek is able to deduce that the Great Animal is a shapeshifter.
From a glance, "Princesses on Parade" is just a simple song displaying all these princesses attending the ball. But it takes on a layer when one realizes: this song is basically Prince Derek's "What else is there" remark come to bite him in the butt, showing him what else is there to Odette these women don't. Most of the princesses in the song are cookie-cutters who are all smile and no personality. And the few who do have personalities are clearly there to swoon over Prince Derek, something Odette never had to do because she already loved Derek for who he was.
Fridge-Heartwarming: All the while the song goes on, Derek doesn't look particularly happy. If anything, he looks concerned, as though preoccupied with whether or not Odette is coming. Not only does it establish he has eyes only for Odette, but it shows some Character Development on his part. As opposed to the old Derek who used to wonder "What else is there", he's not so easily won over by these other pretty faces.
Derek constantly has to learn the Aesop that appearances aren't all that matters — initially citing Odette's physical beauty as the main reason to marry her. This is reflected in the attempts at making the vow; doing so at a ball as a public declaration of love is again focusing on appearances, meaning he was so preoccupied with convincing the public that he didn't notice Odette was being impersonated. In the climax with Rothbart, he defeats him with no witnesses at all — proving that he loved Odette enough to duel with a giant monster to save her.
In Derek's defense, one condition of the spell is that Derek not only has to vow everlasting love, but "prove it to the world," which is why he resolves to do it at the ball with members of every country's royal families as witnesses. But does making a public declaration really count as "proving" his love? By slaying Rothbart for Odette's sake, preventing him from ever harming anyone again, he gives the world much better proof.
"It's not what it seems" is reinforced on the villain's side as well. Rothbart kidnaps Odette and his initial plan is to torture her into bending to his will...only for her to try and escape and save herself. He expected her to be a dainty little damsel who he could easily force into doing his bidding - but she was really a Silk Hiding Steel who would not be beaten by him.
Swans are associated with grace and purity...but are also known for being aggressive and violent when provoked. Odette is a beautiful ladylike princess, but she's also got a strong will and core of iron — making her more like a swan in personality as well as appearance.
Both Clavius and Zelda sing a Villain Song about how the Forbidden Arts are the power to Create, Change and Destroy but Rothbart only uses the power to Change. The reason for this is not Early-Installment Weirdness. Rothbart's lair is smashed and he is turned out with the clothes on his back, completely powerless. It took him two decades to act on his revenge because he had neither the magic orb nor the magic notes. He didn't have the patience to acquire all three.
In A Royal Family Tale, Odette is stated to basically be living and passive Anti-Magic to the source of the Forbidden Arts and this makes the first three films make a lot more sense. Why did Rothbart change Odette into a swan as the coercion to marry him? Because the Forbidden Arts was unable to do anything else to "the Swan Princess". Why did the spell only last one day? It was eventually dispelled by Odette herself and Rothbart had to re-cast it every night. Why did Odette turn back to human permanently after Clavius' orb was destroyed? Same reason but the other non-magical characters attributed it to Rothbart's spell due to not knowing better. Why didn't Odette die at the end of the first and third movie? Because Rothbart and Zelda were using the Forbidden Arts in their attempt to kill her and the Forbidden Arts explicitly can't kill her without the Glowing Stone.
The Curse Escape Clause requires Derek to make a vow of everlasting love and prove it to the world. However, during the ball, it might not have actually worked even if was Odette standing with him. If you pay attention to what he says, nothing is actually directed at Odette, he's too focused on the "prove it to the world" part. What the movie accepts as this vow (whether or not it actually works or it’s just because of Rothbart's death is irrelevant) is Derek just being honest with his feelings about Odette, to Odette. You could even say, Odette is the world. She's certainly Derek's.
More on the Curse Escape Clause: the Exact Words are that to free Odette from the spell, Derek "must make a vow of everlasting love and prove it to the world." Derek and Odette assume that publicly making the vow at the ball will fulfill the conditions. But the spell doesn't say "make the vow before the whole world," it says "prove it to the world." Mere words can be insincere, after all. By risking his life to battle Rothbart in his Great Animal form and slaying him, Derek proves that his vow of love is true.
When Odette and Derek sing about their misgivings towards their engagement at the end of "This Is My Idea," Odette's complaint is that Derek is immature, and Derek's is that he can do much better than Odette. When they warm up to the match, Odette reflects on how Derek makes her feel, and how his smile makes her "see inside him" for the first time, while Derek only takes note of how beautiful she is. The lyrics act as Five-Second Foreshadowing to their split and the problems in their relationship; Derek is still very immature and superficial, whereas Odette is trying to be more adult, and Odette knows she's fallen for Derek because she finally realizes his inner goodness, while Derek doesn't understand his deeper feelings and thinks it's all about her physical beauty.
What makes Odette ask Derek if beauty only matters to him? Instead of asking her to marry him, he just says "arrange the marriage!" Odette has been told "for as long as I remember, we've been told we'd someday wed." The majority of "This Is My Idea" is Derek and Odette objecting to spending their summers together and only having a She Is All Grown Up moment at the tail end of the song. When William and Uberta (who are watching) realize that the pair has finally become receptive to this marriage, they immediately call for a reception to celebrate. Odette hears Derek make decisions for her without consulting her and without someone there to say "what do you think, Odette?" she's got to be wondering if anyone cares about her thoughts and feelings at all. That's what she means when she tells William "I need to know that he loves me for just being me." She wants someone to acknowledge her as a person and not just a pretty face.
In the third movie, Odette truly dies, as in dead! In the first movie, even though she loses consciousness, she might have still been Only Mostly Dead when Derek breaks the spell, but in the third she's burned to death by a fireball! After Derek burns the pages, she is raised from the dead — apparently by the power of forbidden black arts.
From the first movie, where does Rothbart go when he's not in the castle at daytime? Hunting as the Great Animal of course!
Remember how Derek in the first movie initially thought Odette's swan form was the Great Animal? Imagine how things could have gone if seeing that harmless looking swan turning into his lost love only convinced him that the Great Animal was simply assuming her form to mess with him.
