I'm not sure if the Melve Tree really adds much to the story. To me it seems like it should have been a recurring character, but not a regular one. I hope the author will prove me wrong.
I wonder if the witch intentionally chose her to body swap in order to get rid of her magic. Considering what little we've seen of her past it doesn't seem unlikely that she wanted to live her life powerless. This does raise the question of how she found Marietta in the first place, though, especially since they didn't even live in the same country. And there's also the fact that they look alike, could it be more than just a coincidence?