Well she chose correctly and got the knife, didn't she? The Faun "gifted her" a second chance, but did he really? He represented nature, I don't think he really had any say in the matter. He was just pissed at her. And the fairies were steering her wrong, so they got what they deserved
(By the way, was it just me or could even Nate Diaz have leg kicked that Pale Man into bolivian)?
Maybe she also needed to understand the gravity of her choices, so that they really meant something. I think eating the grape was pure defiance of Vidal, who was a glutton that rationed food to his people so they couldn't feed the rebels. She was very hungry because she was sent to bed without dinner after the toad incident ruined her dress, which her mother told her bothered Vidal the most. So, you know, fuck that guy, I'm going to eat from his table.
And yeah, I guess she had to learn humility, too, come to think of it. And sacrifice. The allegory of the immortality rose on top of the thorny mountain thingy told us "Most men are not willing to die to achieve immortality". She was preparing to sacrifice her human form to save her brother in the end.