I’m guessing the Queen of Babel and our archduke have some kind of partnership and are using Ebony for a political maneuver. The Queen probably dislikes how misogynistic the country is and changing up the power structure would make it easier to keep them in line. Since the archduke is the third heir to the throne, it wouldn’t take much for him to be king. I haven’t gotten the vibe that he has that goal yet though.
Anyway, it fits in with how the archduke keeps asking Ebony to value herself, how her situation was unfair, and so on. As for the murders, her father is the question mark. Since they said her husband’s death was an accident, then perhaps her father was not on board with framing Ebony, and/or it was just more advantageous for the Sedells to kill him too?