The thing is, she could f.ck around with the prince all day without picking fights with Lunaria.
Even before the reincarnation, the prince was already into her. In fact, that would be great for Lunaria if she just stole the prince, since it would give Lunaria a reason to push the royal family and Mana would still have the prince by the leash
The problem there is that this would make Mana the next Queen.
And Lunaria would still be a Duchess.
Granted, you can't really depose of a Ducal family and not have repercussions from that act. After all, if the other Ducal families catch even the faintest whiff of something smelling off about this, they're gonna know that there's nothing keeping
them safe. And they
will act accordingly.
But Mana doesn't exactly strike me as being quite so forward thinking. She'll want to get rid of Lunaria no matter what simply because she is the "Villainess", and that's what's supposed to happen in the perfect little world of the otome game. She doesn't care at all that Lunaria is a Duchess or that she has been making absolutely no overtures for the Prince's heart, let alone the hearts of any of the other capture targets. Even if she knew of Lunaria's true origins and intentions, she'd be all, "SEE! SEE! SHE'S EVIL! I'M JUSTIFIED!!!"