I don't think this is the way the plot will go, but what if she really is Elizabeth? ie, she's trying to escape an oppressive older brother and forced marriage by running away, then pretending to be someone else when she was caught? The maid would have to be in on it as the only person who...
Raulus is being clever. He can't interfere with things of this world ... but Yeni's not of this world. So he can poke her into action and interfere with "fate" that way.
I think a few commenters have nailed it. The wine isn't poisoned, the queen let Canaria think that to test her, but not actually poison Cesar. Which means Cesar doubling over is the giveaway that Canaria told him :;