My preferred approach is that the Bretonnian worship of the Lady created an entity slightly, but not entirely separate to Isha/Ariel. Like the difference between Manann and Mathlann or Stromfels. Or Rhya and Dyrath. Or Khaine and Khorne.
So while originally the Lady was something of a trick by the elves, she's since gone rogue and is beholden to the Bretonnian vision of her rather than just being a pawn.
And to the Asrai's horror, it flows the other way too. Perhaps Isha has become a little more like the Lady of the Lake as more of her gestalt power is supplied by humanity.