I actually am convinced with your theory, the duke did say something about it is not the time yet. Nice!
You're actually pretty close. Spoiler for original ending below.
He gave her the dagger to protect herself, telling her to endure. He then goes to the crown prince begging for help, saying he'll side with CP if he does protect his daughter. She does believe her father's forsaken her so she kills herself upon seeing the CP and believing he's there to torture her. The duke then goes crazy and kills EVERYONE. That's how the original timeline ended.
Yeah I also think the crown prince and the duke planned to kill the princess so there wouldn't be a royal who will usurp the throne after the coup d'etat. And now, I realized the prince said those words (when he was going to shadow room(?)) was because the guards are the emperor's people.
I think so too that the father gave that dagger to juvelian to protect herself and I think it's when the time that the rebellion will be held out that's why the crown prince goes to the shadow room to set her free but she killed herself bc she thought her dad abandoned her but in the end I think the rebellion was useless bc the one thing that the Duke wanted to protect was dead.. DANGG