It was clearly explained in this chapter, pay some attention.
Nobody wants the marriage to happen. Calvino doesn't want his daughter to be used as a hostage and Serranova doesn't want the Calvino alliance to go to the Aldinis instead of them. He offered them honestly a good deal. Lose a little money and power to prevent a disaster scenario peacefully. It is literally all a power dynamics thing that you claim you want.
Well it's clearly not a good deal. Much as annulling the marriage is clearly what all of them want from our reader perspective, Leonardo is presenting an impossible deal to the two other heads.
Give up prized portions of their own respective family's empires, not to mention enriching the Aldinis at the same time, or stick with the marriage and Leonardo gains much the same through political connection anyway.
I think you read too much into Leonardo's words claiming it's a small price to pay. They definitely aren't. He's being sarcastic, remember he's not using his "good guy" face here.
I'm not saying of course, that the marriage is something equally important an issue. But that's why it's an impossible situation.
My current read is Leonardo actually does want the Serranovas and Calvinos to go through with their plan to annul the marriage using the ancient protocols. However, he's concerned that Francesca had gotten to close to him and might reject the Serranovas proposal. He's giving both families and Francesca no choice but to go on with their plan.
Remember, annulling the marriage himself is an empty offer. He's bound by the same rules the Calvino family is. He's talking about breaking the blood pact himself, which the other two heads won't seriously believe. If he instead intends to force the duel to happen, things will play out same as the game. And furthermore, it would also prove that he wasn't actually a villain in the game either, which is why he was intended to be an obtainable character there.
If we want to consider the usual shojo trope someone mentioned about "I can fix him". I would remind that usually the case in these shojo stories is not just the protag thinking "I can fix him", but also that the villainous male lead turns out to not need fixing at all.