what you remember is essentially correct regarding what the mark actually is: the mark of the "chosen one" is something related to the spirits, and something he can apply to "anyone" (slightly more strict, but you get it). The phrase more refers to how the person with the mark is chosen by the spirit contractor.
However, the prince said it in a way (in the game) that made it sound like the "chosen" refers to chosen by fate/destiny. That there is no other than her that could receive the mark. Pretty much he fooled the heroine (and thus all gamers) because he felt it was more romantic that way, and wanted her to feel special. This is why both heroine and mc thinks he cant be happy without heroine, no matter what his actual feelings are.
And yes, she forced a lot of stuff (and this is another reason why she is so assured of her status as a heroine). Before the game she noticed her backstory went as the game had said it went, she encountered the mascot bird and saved it, and so on (as I said, iirc she was unaware it is a spirit, and as such that it was Charming everyone. That mc also had no idea about spirits implies this too). The bird is charming everyone and she thinks stuff is just going as the memory said it would, and that she is so special that anything she wants happens. So yes, she is definitely seeing what she wants to, that is my entire point behind how external validation made her blind to the incongruities. Not to mention the prince playing her to orchestrate this scene.
Essentially it seems we are saying the same thing here: she is a poor girl (well, I claim the former part, the following is more where we seem to agree?) who fell victim to her preconceptions and people reinforcing them. The only time the preconception was very clearly wrong (the ppl hitched up with others) she accepted it (except for prince) but had her worldview for the most part unchallenged as even in the game the captures were not set-in-stone.
The "oracle" I refer to, is the memory of the otome game. We can call it anything really, future-vision, oracle, prediction, narrative or memory of the future, I prefer 'oracle' tho.
And I don't think she is an official saint no, but she strikes me as someone who believes themselves to be god-ordained to save the world. I don't know any other word to describe such a person than "saint" (hero perhaps, but feels a bit off). Consider it, she is born as the actual heroine from an otome game, the world bends to her every wish and whim, beyond even what the oracle explicitly said would happen, as if she some higher power wills it so (cough you might want to investigate your bird, heronia cough), the prince says (in the game) that she is literally the chosen one (and any route where he doesn't say that, he either ruins the world or himself). What other conclusion can you draw in her shoes?
edit: Yeah, as the father says, he really should have corrected her earlier. And not (presumably) simply doted on his precious daughter while charm-addled.