Japanese authors may sometimes have a weird perspective on European history, be it Middle Ages, Queen Victoria's ruling, or introduction to French Revolution.

Ironically, the downfall of this manga is caused by its indulgence and excess--the very things it portrays as leading to the downfall of the French aristocracy. You see, especially in the latter half, it indulges in symbolic and metaphorical pictures, as well as snapshots of potential futures or poetic representations of events. (I noticed at least twice when they had characters presumably singing their internal thoughts). All that means that each chapter has very little actual content.

The character of Mary from a free-falling sociopath becomes some sort of analysis on gender roles, LGBT history in 18th century france, polyamory, racial discrimination all in the span of 10-20 chapters. It's so on the nose it's almost funny.

Charles had a slow transition into the character of his father that went slipping down at the speed of light as soon as the author decided to focus on Mary, all she has to do is drop random quirks about how he changed as he whines about family obligations.

He tortures his own son. His opposition to torture set the basis of his character from the beginning, it defined his values and worldview to the point where he was ready to topple the seemingly endless power of his grandmother. It led to his liberal sympathies. it was the central reason he didn't want to kill people violently, which was what would have led him to seek out the guilottine.

The reader is never given any reason as to why he changed, he just did. There's two comments about the world being rough and how raising kids made Charles more conscious of his family. Which could certainly be explored as a reason for him to enjoin with the system. But here it's a magical incantation that transforms him from a grey, morally tormented character one page into a saturday cartoon villain the next. Replete with last minute d'oh moments as his dastardly plan to have his sister married is befuzzled scooby-doo style.

Everybody gets simplified, flanderized, almost a caricature. At the start it was beautiful, and read almost like a novel. The father, grandmother, uncle were all "villains" but were given ample space to prove that they had a 3-dimensional personality and were acting out of selfless motives but constrained within a hierarchical system that forced them to participate in violence, which hurt them as much as the victim.

When father Sanson tortured Charles. He acted out of a belief that the firstborn must inherit the family business, and therefore has to be strong in character to take a life. Something that can only be achieved by getting one accustomed to violence. Said torture also had a secondary purpose of forcing compliance with the unreasonable demands of his family, such as the inheritance itself. He knew it was painful for the son, but justified it by claiming that his son was his flesh and blood and an extension of him. Despite voluntarily participating in child torture, he is a victim as well.
What could be done to fix this?

Removing constricting family institutions, authoritarian systems, class-based understandings of occupation, societal acceptance of child violence, patriarchal norms. For there to be no Jean, the whole political, economic and cultural landscape has to be changed from the grounds up.

When a bunch of white french aristocrats attack a half-black french noble for being mixed, then burn down his orphanage with the children still inside, provoke him to a fight, kill him and publically leave their family emblem in his body. They did it because they were assholes of comedic proportions. Even with all the discrimination present in pre-revolutionary france, burning down an entire orphanage wasn't something people laughed off.
What could be done to fix this?
"Literally, like, stop being evil. "
That's all the message there is. No compulsion from a higher system, no psychological explanation for their actions. The reason they burned kids alive was because they wanted to burn kids alive. The way to stop them from burning kids alive is for them to stop burning kids alive.

The art is admittedly well-drawn. Though, to be honest, it might be too ornate. The lips are overly lush and shiny. The backgrounds are grainy and at times there's too much going on, making it hard to focus on the relevant details. Also, sometimes characters' eyes look badly placed on their faces--like they're just tacked on. But even if the art was perfect, that wouldn't justify adding needless pictures. If this manga had trimmed all its padding and perhaps been only about half as long, it would've had the potential to be a masterpiece.

The last half of the manga introduces Marie Antoinette into the storyline and begins setting the stage for the events which lead into the French Revolution. But that's all that it's doing: setting the stage. The entire last half doesn't really feel like it has much purpose other than acting like a prequel for the next manga.