On the Duke family, at that point Reynold and the Duke try to get close to Penelope to heal her heart after a case where they did not trust her.
Derrick the pervert jealous and got manipulative, even trying to attack Eclis/the prince/Vinter to keep Penelope. Penelope fear these behavior.
The dumb prince use royal power to force Penelope to meet him, and Penelope show Eclis that she dont want the attention from the prince. She also regret a lot when their date got cancel due to those "forced invitation".
To Eclis, the Duke house is oppressing Penelope, with a pervert brother lusting over his master, and it seem like the Duke want to get close to Penelope to marry her off to the bloody crazy royal prince in a political game.
He believe destroy Penelope position will make royal prince and Duke house to throw the commoner away, while he have the Swordmaster rank will cover for her and get everything he want. Technically, if Penelope actually love Eclis, this may be the Ending for Hard mode Eclis route, if all the relationship not changed.
But, the real life situation are not game, and Eclis do not understand Penelope situation at all. She also do not trust him enough to tell him about her intention or how close she is to her father/prince, think he want to kill them. Both Penelope and Eclis hide their own knowledge about his "enemy kingdom's prince" identity, which lead to many wrong assumtion.