Einrich did the good ol' engagement cancelling scene, which ends up backfiring on him because it provide an easily understandable and dramatic stage.
Serena steps in to defend Rosemary (and the family). She points out the engagement is something between families, so even as a prince and with reasons, Einrich can’t cancel it on his own, and definitely not with this kind of public humiliation. If he wants to cancel it, then he must speak to his father (as head of his family) about it.
Serena also points out, Rosemary acts in retaliation to Einrich’s own behaviour, and as engaged man, he should give more attention to his fiancé (or at least, not being publicly intimate with other women).
She demand Einrich to pay reparation for this insult. Not the royal family, but Einrich himself, to put emphasis about his position. Unlike Evan, he is not allowed to use the royal family name, and can’t touch the royal treasury.
He ordered his guards to arrest Serena, but the argument is on her side. So Einrich pull his sword, and that serve as justification for Evan to interfere and arrest him.
Duke Violet made a complaint about the insult, and use this as pretext to demand the engagement to be cancelled. The King even lowered his head to apologise. Einrich is removed earlier from the royal family and is known as the person who bring shame to the king. He’ll be troubled to get married now as nobody want to associate with someone the royal family clearly don’t like.
Einrich’s mother tried to get back at Serena by sending assassins, whom she kindly return their head back to the sender.
When the concubine tried to harass the Violets with her family’s influence, it backfired as the merchants would rather side with their business partner.
And when she schemed to use monster calling item and heavily injured Evan, Serena had enough and decide she should have killed that old woman much earlier.