I think the reason why Astelle was the one accused of being the illegitimate child is because her mother was said to have an affair after marriage. So her mother could already have given birth to her first child.
In the novel,before Jacqueline was married to the duke of Reston she was said to be pregnant that is why when the duke found her diary,he got this doubt under his skin that his son,astelle's brother is not his'. Jacqueline was sold to the duke even before Astelle's brother was born,so why doubts Astelle's birth now? Ecklen was in the war after Jacqueline's marriage,even after her son and daughter were born Count Ecklen did not show up. Actually in the novel it was Fritz's birth that was questioned,the reason why the duke of Reston wants to marry again. Iirc,he got his interest in Florin and the older sister who was the first villain in this story. But he was poisoned by Kaizen and died. Belian was sent north for helping Florin. Actually none of this happened in the novel.
How can a low noblewoman and a daughter of a criminal can accused a powerful empress like that? She even managed to walked inside the palace ground and talked to the emperor? Wasn't it to arrest her first and put her in the dungeon for being a criminal? Besides,how can people believe in her testimony when it's very obvious that her family had done a lot of crimes? An empress without a single flaw and a daughter of a criminal,which is more believable?
I'm amazed by how easily the people here believe in such accusations in one day. If there is a scandal between the late duchess and Ecklen,the one whose birth would put in question is not Astelle but her brother. This is the first manhwa I've read where the common people went to the palace to accused their empress. Isn't it laughable? And about that poisoned issue of the late empress I'm sure it's Belian. In the novel, nothing here now happened but we'll,they want to change it.