(masking this just because most of it is a rant) And most importantly, it's clear that the Marquis knows more than he's letting on and his inaction is a deliberate choice that marks him as no innocent in all this. It always grinds my gears when the lord of the manor who sits on his ass while horrible things happen under his nose gets treated like uwu cinnamon roll just didn't kno bad lady would do bad thing, as if he didn't grow up in the same social environment whose pressures produced those "evil women" who, yes, do terrible things, but nearly always in order to secure safety and comfort for themselves and their children in a world where death is cheap and not birthing a son is enough reason to be thrown out to starve or be conveniently poisoned or framed or whatever while the lord looks the other way, because god knows women have no legal protections in the absence of men's favor. There is literally no way to hide all the power struggles happening in your own dang house! All those men bin knew!

he is very favored by his uncle, the emperor, but still level-headed about the whole thing. He is loyal to the emperor. To show that he doesn't have any intention to rebel, he is even willing to get the imperial decree that states he is marrying into mc's family even though it can cause people to look down on him

they got together in the middle of the book. ML is clear with his intention to marry MC. It's nice, mc's obliviousness doesn't get dragged for too long

he was the ultimate example of a safe-official. His brother was a very greedy person and even took his share of inheritance left by their parents and married him off to a daughter of a butcher. He didn't really like his wife but he got along quite okay with her. Later he found out that his brother and sister-in-law had a hand in the disappearance of his pregnant wife. He was a new official at that time, his brother, albeit abysmally, did raise him. If he made a ruckus he surely would be accused of being unfilial and his official career would be over. So he didn't do anything and he ended up treating them indifferently.

he also treated his wife and concubine with the same indifference. He was quite nice to his children at the start. But since his concubine's sister gave birth to the second prince, he started treating them indifferently too.

basically, he is only loyal to the emperor and doesn't care at all about anyone else. He doesn't want to be seen colluding with his concubine's family who supports the second prince to replace the sickly crown prince. He knows zhuang zhong isn't his biological son, but he still registers him as his di son to hold back his concubine's children. He would rather a complete stranger inherit his title than be seen as favoring his concubine's family even though the only children he has are the shu children from that concubine. He is truly amazing, really ruthless lol. After everything was over, he resigned and left to be a monk. I guess he is really tired with his life.