The point of having Marguerite not know that she would die too if Jean lost was to show that Jean couldn't care less about protecting his own wife from everyone against her, which made the audience sympathize with her more. And she only felt that she should have remained silent because now she had a son to think about, I'm pretty sure she even said as much. In contrast, Jean just kept his distance and didn't so much as touch his son before he left the room. Again, this was to make the audience sympathize with her because she wanted to do what any good parent would do and put the needs of their kid above their own.

I also think that making Marguerite's POV the truth was the right call. While we don't actually know what happened, I don't think it's a stretch to say that Marguerite was telling the truth given that nothing has openly contradicted her. And seeing that this movie was really about Marguerite as a woman standing up for herself in such a male-driven society, any ambiguity would probably undercut that theme. So ultimately I thought her character was pretty well done, but these are just my two cents.