I honestly think this manga has everything written rather plausibly.
The 1st prince, original fiance, married for political gain, wanting a wife with use tied solely to family backing and not anything like Eliana's smarts. He didn't bother checking the sister and only realized she was potentially negative in the useful points after marrying her and getting pestered every day.
The court mage who doesn't believe in dragons... don't think he's ever left the country, and he's so archetypically power-hungry and focused on a high position that he's blind. Self-centered people who tunnel vision on power are very much capable of being this stupid.
@Metazoxan The king, Eliana's father, and really, all the nobility- they DO have reason to hate Eliana. It's self-centered, but they DO have motivation- it's cause it looks bad. They switched out an engagement like it was no big deal. Screw all with proper procedure. Now she's useless, in many people's eyes. What are they gonna do, make her marry the other prince? She's just this unmarried lady that might hold a grudge. She's smart and useful and capable, you say? That's dangerous, woman smart, me dumb? Witch! In a family- no, in an entire royal court where peanut brains are in power, who would value a capable woman? Think about it.
Sure, they're all dumb, but the reasons for their actions are not simply because they are dumb. Image consciousness, self-serving-ness, paranoia, etc. People project their own paths of action onto others.
All the little things they seem to be going against Eliana for are just convenient lies and excuses. That's why there are so many variations. "She's a disgrace, weak, shameful, didn't wanna marry the dragon!" "She's dangerous! She has the power to kill me/us!"
And "neighbor kingdom" is a little deceptive. It's pretty far. Plus Greed took it over in a day. So like.
To be honest, the most unrealistic thing in this whole mess is that this genetic pool of selfish celery brains could for some reason be able to nature OR nurture their two (2) nobles of any caliber (Eliana and the first prince). And even then, they lack a little common sense, I suppose. She, First Prince, and selfish-Chris are intelligent, but they too are affected by the kingdom's culture and procedures.
First prince did things for political gain and bit his ass for it, but that's a totally natural thing. He wasn't at fault for cancelling the engagement, just had a momentary lapse of judgement for not background checking. Eliana is unnaturally naive, but she was raised to be a tool. Chris is just a selfish wuss with stalker tendencies, but yknow, pales in comparison to sister and the rest of the court.