
Oooo I feel like I know what you're talking about. I think it's because it doesn't feel realistic ? Like even if it's fiction, it doesn't make sense according to the world building the author has done for the manga. Like even if it's make believe, the author wants to set this vibe but the things that you mention doesn't help. It ends up feeling out of place.

2nd prince is probably the son of the empress/1st queen/main wife. Thus even though he's the 2nd son of the king, he has more legitimacy and supporters to the throne. Excluding his personality and achievement, of course. Usually those are taken into consideration too, unless the country in under tyranny.
Why is everything so casual?
Previous country:
You can sell a saint just like that? Where's the king? Is the 2nd prince the crown prince?
Current country:
Again where's the king? I know some might say nothing really happened for him to get involved but a literal SAINT came from another country and they don't have to greet the king and queen? Such crisis happening and the king doesn't have to be present for the countermeasure meeting?
I like it so far but it's kinda floating ? (〜 ̄△ ̄)〜 (dunno how to convey/express it into words about how I'm feeling about this)
And does it have to be the 2nd prince to be ml and ex fiancee? I like the 1st prince more (he's prettier ) well it doesn't matter as long as it's no once from her previous country