Ok, first off, Merlin’s awesome 🙂 The back story is a little silly, but oh well.
Second, yes, absolutely, I think DO=Merlin’s lost love=Nimue. I also think she wears the mask not only because we’re not meant to know who she is yet, but because she has gone scaly/sparkly.
So, going back to the whole “Merlin bound the darkness to the dagger” I wonder how that factors into this backstory. And if he couldn’t save his love (i.e. banish the darkness from her) and could, in theory, for Emma, what’s changed?