I actually gave this some thought years ago. Assuming Eric was gone, but Ariel was restored fully as a mermaid with her voice, what would become of her?
It was pointed out to me that banishment means being cast out from society not just physically, but also emotionally and socially. "You are not welcome here because of who and what you are." So, despite all the trouble she caused, Ariel is still a princess, and that could very well be the only thing that keeps her from fully having the book being thrown at her by her own father despite their kinship. Legally she may have to be punished for all the trouble she caused- undermining the laws, putting the King at risk, aiding a known criminal, going to the surface, consorting with a human, potentially exposing the existence of merfolk everywhere, etc etc etc- she is still just immature enough and of royal blood to not be basically executed for criminal mischief. Therefor, let the punishment fit the crime. To be banished or executed might be missed on Ariel personally because she has had now-two things very dear to her taken away forever- permanent humanity, and her love- and she will have to live with those losses for the rest of her roughly 284 years of life. I don't care who [or what] you are- those are deep investments that have been ripped away from Ariel and will haunt her forever.
So, let us assume Triton does not banish, execute, or jail Ariel. What then? Well, this is what I came up with:
Because Ariel is a liability to the palace (at the very least), but is not set against the society that raised her and that she dwells in, it may be best to remove her royal title but continue to allow her to continue to live in Atlantica. She's now a commoner, a loyal citizen of the King and his six beautiful Princess daughters.
Ironically, in another time and place, Ariel might be called otaku. She is rather unique amongst all merfolk in that she knows more about humans through practice than anyone else. (Of course we would know better, but who's to argue down where it's wetter?) And she is also not afraid to go into shipwrecks and collect human things (even if the grotto was destroyed, there's more where that came from). I imagine, once she overcame her immediate grief, Ariel might get it into her head to share the things she learned about the surface with others, to prove her point. Not to usurp or overrule the King's authority, but to show what she had been advocating for years- that humans aren't as bad as they seem. But, she would need a place to stay, and to work. Howsabout creating a 'Little Shop of Human Horrors'? A little hole-in-the-wall museum dedicated to showing off curiosities of the surface world in a safe environment, with a former princess to show you around? "Come and see the strange dinglehoppers and oohzits with a confident and knowledgeable mermaid as your guide!" Yes, Ariel could become known as the odd and imaginative curator of of Ye Olde Curiosity Shoppe Under The Sea!