Nyxmay 100% had to have come from Viridice, however, I'm not so sure she's the former guardian. I think it's implied more that whatever she ruled over was a secret place, a utopia that was shattered by her mercy, her altruism towards that adventuring party she rescued who likely came back to take and slaughter the fairies for profit most likely. I'd think a former guardian would be well recognized, and she doesn't look too different in the dream sequences, and she didn't change her name either.
I personally think it might be either Viridice, or a still unkown isolated location. First one would explain vegetation and all, also the implications of how Nyxmay became evil show clearly, that it will be something cruel and related to faries, and that's why we've never seen them in the present. However, the fact that everything happens near an ancient temple or something, which was never mentioned by anyone who lives in Viridice, might imply that it wasn't this or any other kingdom. Also the fact that faries literally know no violence adds to the theory of isolated environment.
And yes, the fact that we don't know when that happened, there are good points why it's possible that Nyxmay might've been a guardian of Viridice and then left. That, however, brings questions like why fairies were calling her "Queen", if she expelled herself, why she stayed somewhere in Viridice, and most importantly, if she meant a lot to Regina, in fact, so much, that she couldn't take any insults of her mentor, she never told even a little bit about her to MC, especially on affection route?
