Okay, so first... YAY! They switched back, and no cute-little-animal-girl-MC is a stop closer to marrying what's his name (unless violet decides to marry her; she's the type that just fuckin might...).
Second, the poison sounds like a binary/trinary chemical compound, one that likely needs to have magic present for it to become poisonous; if I was writing this, I would have it be something like "powdered demon stone," or some shit that only becomes poisonous when it is combined with one or two other things, and then is exposed to magic; since every person has mana, the act of digestion should be enough for the powder to become charged, then combine with calendula and oranges and then form some kind of Strychnine (this totally doesn't make Strychnine, I'm just making shit up, FYI).
Hell, if the orange cookies are made with some of the Ergot of rye, you've good reason for hallucinations, right there...

Again, that's what I'd consider as a writer; as the
actual Villainess Antagonist, Lilly, I would probably have just used candles that have the wicks laced with Strychnine or some kind of cyanotic; I mean, just make sure you're near the window (to breathe), while you watch him expire.
Finally, I
really... and
I mean REALLY, desperately hope that grandpapa Argonuts has not known about her treatment, and has a high-speed-come-apart about our darling MC's treatment at home.
I mean... I really, really want these shitty people put through the wringer.
Some more, I mean; through the wringer even moreso; publicly; maybe with a forfeiture of any and all belongings, funds, titles, estates, personal effects... they can have one outfit, and one threadbare, moth-eaten blanket.
And that's more than they deserve.