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You've indicated that Nimue was the third daughter of Igraine and Gorlois, exchanged with the changeling Morgana. Does that make her magic (or that of stolen children in general, if you prefer) mortal sorcery by definition? Except mortal sorcery isn't allowed on Avalon, where stolen human children would logically be raised. Did she not know any magic before Merlin taught her (as in the legend) despite her upbringing by the Third Race, or is she (or stolen children in general, if you prefer) able to manipulate the energies of Avalon like the Magus learned to, thus technically staying within the prohibition?
Mostly, I'm going to answer: NO SPOILERS. But I will say that not every option you cited above is mutually exclusive with every other option.