An easy to miss detail if one doesn't have the close captioning turn on is the fact that Elodie's father is not a king, he is Lord Bayford. This not only plays into Queen Isabelle's contempt and dismissal of Elodie as a commoner later in the movie, but make much more sense on a deeper level: it will be far easier for the Aureans to find noblemen whose fiefs are in financial difficulties and thus be open to trade a daughter for a large quantity of gold, far easier than it would be with the daughter of an actual ruling sovereign.
Also, there are far more lower ranked nobles than there are kings which means more families to draw from. And daughters of kings tend to be more well-known than daughters of earls and lords; their disappearance would attract less attention from a political sense.
Unlike most depictions of dragons, with vertically-slit pupils, the dragon of this film has horizontally-slit ones, like a toad or a goat. Horizontal pupils are a trait primarily found in prey animals, as it helps them see behind them: symbolically, it could be a visual cue on how the dragon in a way was a victim, "prey" for the royal family's cruelty.
Given that Queen Isabelle was born a princess, that means she most certainly witnessed girls of lower stations than her like Elodie be sacrificed in her place so she would be spared. It's no wonder why she's so classist if she was raised by her parents to see those girls as expendable and herself as not.
It makes sense that Queen Isabelle doesn't remember Elodie's name so shortly after meeting her, as she's also recently met the first bride and possibly even the third already. They're bound to blend together, especially since their sham weddings to Prince Henry are identical.
This is also likely why she was so quick to reject Lady Bayford's attempt to befriend her (aside from her classism, of course). The royals can't risk getting too close to the oblivious members of the families whose daughters are being sacrificed, as then they'd risk having said families catch wind of the others and realize that their daughters aren't the only ones marrying the prince.
By comparison the dragon takes the time to learn each of the Princess' names, how to say them and remembers them. The Dragon cares, in a twisted way, much more than Queen Isabelle
Why does the dragon demand a sacrifice of three daughters once every generation if she had only one clutch of three hatchlings that were killed? Considering how the dragon is implied to be centuries if not millennia old, it's possible that her perception of time is different than a human's or that she sees a human's lifespan as insignificant compared to a dragon's. Her daughters would’ve lived for hundreds or thousands of years, but they were murdered by beings who live for a relatively meager 90.
On the surface it might seem strange that a dynasty as ruthless as the Aurean royal family would go through all the hassle of procuring foreign brides at great expense in bride prices to be sacrificed instead of just rounding up local peasant girls, considering that the ritual doesn’t even require the marriage to be consummated. Closer examination shows this to be more of their Pragmatic Villainy at play: sacrificing peasant girls might save them a lot of gold, but in the long run it risks either the commons deciding to revolt or worse… ratting them out to the dragon. The movie itself shows that the island's commoners are perfectly capable of going in and out of the mountain if properly motivated.
Logically, since the woman Elodie sees at the palace is another bride sacrifice, the prince she marries should already have a cut on his palm from the last wedding.
Maybe the royal family knows about the gloworms. It would be an additional source of wealth.
