It would definitely serve her well to leave as soon as possible. She seems to believe she knows what the marquis is planning, but at the end of the day, he's still a man who just lost his only daughter. He could do anything, and not even anything sensible. So, if she wants to leave, she better do it asap. It might be risky to try to face the marquis. She told the two stooges the marquis wouldn't even have time for her, but I reckon he might instead demand things from her that she's not comfortable with. While the marquis and marquise might have had little attention to give her while Ophelia was still around, they might think exactly the opposite now. If they demand she stay and marry Raretis, what is she going to do? It might turn out nasty once the baron's daughter appears, if at that point Raretis turns his eyes on her.
But I guess this story would get nowhere is she hits the road and leaves the whole mess behind.