For me you have a point somehow but, let's not forget that the kid is acting nice purposely when she's around? How would you think that innocent face like that have two face character? And from the very start she thinks that the Duke is not a good person so why would she believe the person she can't trust? And about the poison incident even if they told her that the kid is taking that poison everyday she would think that he was forced to do that. Personally I like kids a lot, call me a idiot or what but if I were in her situation right now I will believe that he's innocent especially if I will saw the innocent face he's doing right now. We can all just say this right now cause we knew the situation already unlike her. But for a reader's point of view I would say that she's really making herself idiot right now for getting fooled like this.
The female lead is kind of annoying, like I know she wants to raise him with love so that he doesn't become a villain but she it seems like she doesn't know him at all! Like people look at you shocked every time they hear you call him innocent the first time and the Duke himself straight up told you the kid isn't normal yet she still doesn't listen! She doesn't even ask him what he did to say that, just assumes that he's talking bad about his kid for no reason. Like come on lady, does he look like the man to do that?! Also, she got so upset over the fact that he was drinking poison that she forgot that they told her he took it DAILY meaning everyday that she's spent with, he was poisoned and didn't act different so why are you flipping out now?! I lowkey hope that he does get poisoned (Doesn't die though) just so that she can see why they were doing it and how its important because she's obviously failed go realise how disliked her family is and how many people try to kill them.