I actually like how they did it for this because her pushing her bias and the FL together is because both his life and his right to rule are directly tied to that route in the game. She does it out of a need to protect him and not just because of arbitrary "well they end up together in the game so I should get them together".
Strictly speaking Alex dies when Onyx's route is taken and he only survives if the FL takes his route and goes on that journey.
Obviously, stuff changed but only because the witch hastened the curse and now MC was able to find the place herself and cure him, but I think it was reasonable to push toward getting them together.
Now the problem is that him becoming King is jeopardized and only marrying the saintess let's him become king.
I also like that she has a history of being the supporter for someone else's love rather than her own. She has actual reasons to not even realize she's an option on the table because she has always been sacrificing her own chances at love to make someone else happy and she was happy to do it too because the results made her happy for the couple.
It just doesn't register in her mind as a possibility. It's like rooting for your favorite Kpop idol: you support them, but they're so far out of reach that you'd never possibly imagine them liking you.
I just think, compared to some stories, they do a good job of having stakes for if leads don't get together and show us pretty well why she acts the way she does.
Also, I would argue the characters are not attracted to the player (at least in this story) because of their decisions. Onyx is a romanceable character (arguably the MAIN love interest) and he absolutely has no romantic feelings towards her even though she's the player. They're now in a fully realized world that isn't restricted to just game logic. They are able to be more complex because they are now real people.
One thing those who transmigrated into an otome game never thought is that even if they were playing a character in the game, whatever route the character chose and who they ended up with was all done by the player. Things happened because of their choices. Predetermined plots were there but nothing was set in stone, the player's choices still directs how the story flowed, which means the player was truly the love interest of the capture target, wearing the face of the game's main character.
Characters like Arianel were so obsessed with recreating the scenes that were created from when they played the game and never realized they were no longer the player and this world was no longer a game. At that point, they're imposing feelings onto other people for the sake of their obsession.