You're more or less on the mark in your assumption. In the game Bertia and Heroina are familiar with, the royal family sometimes have prodigies like Cecil and their soul crushing boredom ends up being potentially quite dangerous. They will latch onto the first thing they find truly interesting and become obsessed with it as the only thing that provides a feeling of contentment. So if one were to become fixated on the art of war, they're going to plunge the continent into endless war to indulge in that; thus Bertia's kneejerk reaction to Cecil being intrigued by gunpowder in this chapter. IIRC the founder of their country was of this nature and built the kingdom as part of his obsession, which was luckily just love for his childhood friend. Cecil sort of gets a bad ending in all storylines except his own since only the heroine can become something he benevolently attaches to and ultimately learns how to love and be kind from. So if the player goes to any other character ending, the happy endings always have a lingering shadow of doom floating over them, since it implies Cecil is going to bring the kingdom to ruin in pursuit of relieving his boredom, since this problem can't get resolved due to the heroine not getting involved in Cecil's storyline.
This is why Bertia is fixated on ruining herself and ensuring Cecil ends up with Heroina, and why Heroina similarly asserts she's the only correct option- both are running with the game script assuming it's set in stone and that the game mechanics still control what is now the 'real world' to them (even though Bertia has materially gone off script multiple times- averting the plague that was supposed to kill her mother and cripple Cynthia for example, proving that while most of these plot beats are going to happen, they can be altered or averted entirely, since nothing else happened to ensure Bertia's mother died and her father turned evil after the event where that was supposed to happen resolved differently). What we as the readers know that Bertia and Heroina haven't noticed yet is a lot of this had been averted 6 years ago because Cecil had already found Bertia to be one interesting thing in his life ever since their first meeting- concepts like 'fated maidens' technically aren't real in their world, it's just otome game jargon they're putting too much weight to. Cecil himself isn't quite aware of his own nature in this regard either and is more than a little stilted in understanding his own feelings, so even he's only just now realizing he cares for Bertia beyond social formalities as merely a beneficial political marriage he originally accepted based entirely on emotionless logic and reason.
