Considering how the duke has been depicted since the beginning, coming from his mouth, that doesn't even sound like a plot convenience. He's a man of extreme principle, but those principles aren't dictated to him by someone else, it very much seems like, rather they must be largely his way of life and his view of the world. Just like he refused to execute Clarisse because she's underage and even went as far as is providing her a splendid childhood, regardless of her impending capital punishment (which I imagine will only stand as long as Clarisse is the princess of the vanquished kingdom. Should she become, for example, the daughter of a certain duke, she wouldn't anymore be the princess and need to be executed). Similarly since he accepted the fake duchess as his wife, he's not going to change his decision so easily.