It's not that. In one of the route it showed that the og fl took him to the flower field that cured him. Plus she knows how he would make a better king so she wanted to pair them together so according to the flow of that route the ig fl heals him and he becomes king.
She did step up when she realised that it won't be possible so it's not like she is stupid just rather than seeing this as her real life she is treating it like a game with set routes

At first it was kinda funny but the FL is actually pretty annoying. Like… she was 30 in her first life and acts like she’s 12. And why is she being dumb thinking Alex will die if Marin dates Onix, when she KNOWS he’s sick and that’s obviously what’s gonna kill him anyway?? When the humor feels this forced it’s not even funny, just painful to read