WMG / The Swan Princess

William and Uberta were both in arranged marriages that didn't turn out so well
Maybe they were in love with each other, before marriages were arranged with other people, maybe they just both ended up unhappy in their marriages, but whatever the reason, William and Uberta were in marriages arranged with other people, and the marriages weren't the happiest ones. This is why, when they had kids, they were willing to spend essentially every summer trying to get their children to get along, instead of just shoving them at each other at the age of majority, and telling them that they were engaged- they wanted their children a chance to fall in love with each other.

Of course, they probably didn't foresee how much they would fight with each other, or anything, but they at least wanted to give their children a chance.

Derek and Odette planned their whole relationship from the beginning.
After the initial meeting, the two made up and decided that they liked each other, and that getting twice as many subjects and wealth when they grew up was a sweet arrangement. However, they pretended to fight and not get along for the next decade or so to troll William and Uberta. They were in a serious relationship and intended to get married since their teenaged years, and planned the whole "Is beauty all that matters to you?" incident ahead of time for fun. The next step in their plan would have been a big reconciliation, but Rothbart kidnapping Odette screwed that up. Once his lifelong friend and lover was taken away, the gloves came off and Derek stopped pretending.

Adult Odette is an imposter.
The actual Princess Odette that Derek spent his summers with died of illness or an accident, so King William adopted a niece or young cousin or something and passed her off as the real thing the next summer. Derek and the Imposter Odette already knew and liked each other, but were unable to be together because he'd been betrothed to the real Odette all his life. When they replaced Odette with the girl he liked, Derek went along with the operation enthusiastically. However, he got tongue-tied when the Imposter Odette asked him about his feelings for her, and gave her the wrong idea that she was just a pretty Replacement Goldfish for the real Odette. After Derek went to such lengths to free Imposter Odette from the spell, however, she realized that it really was her that he loved all along and not the real Odette, and the two never told a soul who she really was.

Jean Bob really is a prince.
Why not? Rothbart can turn people into animals. Given Jean Bob's general attitude, he was probably abducted as a tween, turned into a frog and lived there for quite awhile, likely presumed dead by his people. He's foreign enough that it wouldn't compete with the rest of the plot, and otherwise—how would a random frog know French? Speed's probably still a turtle though.

Derek really did break the spell upon Odette at the ball.
Rothbart could have added another spell so that Odette would die if Derek had made the vow to the lackey who impersonated her, but he may have kept the original spell on Odette active, thinking that it would not come true at all at this point. What Rothbart might not have realized was that even though Derek thought the lackey was Odette, the real Odette's presence at the ball when Derek made the vow would still be enough to break the spell. It's also possible that the spell breaking "cancelled out" Odette's death (a la Beauty and the Beast), and Odette merely passed out rather than died in the scene before Rothbart and Derek had their battle.

Why there are never any guards in the castle
We never see guards in the castle. Clavius can basically walk in and lock Odette into a room inside the castle and no one stops him; this is before he has the power of the Forbidden Arts. All the castle servants are just that, servants; cooks, cleaners, tutors etc. In A Royal Family Tale, the flying squirrels fly in and immediately have the full run of the place. Why is this? A Royal Family Tale states that "the Swan Princess" will usher in "an Age of Goodness". Indeed, just Odette being born caused the Forbidden Arts such pain that it retreated to its orb and was so terribly weakened that it couldn't take direct action for decades. Just by being alive, Odette drastically decreased the power of evil. Now recall that all of the villains in every movie are in some way connected to the Forbidden Arts; either practioners of it or their minions. This leads to the main guess: the only evil people in the setting post-Odette's birth have been corrupted by the Forbidden Arts and for this reason neither the king nor queen of two separate kingdoms saw a need for guards. After all, why would anyone need guards in an "Age of Goodness?"