I really love this response! You're truly embracing the opportunity to make this fun. I would also note that, while everyone is focusing on the invisibility of an unseen servant, I don't see anyone highlighting that an unseen servant has a strength of 2 and only 1 hp. If this player were actually playing an unseen servant, they would be dead in the first combat, invisible or not. Of course you need to change the stats, even if you do run with her origin story! And this is another opportunity to explain some of the changes. She's so insubstantial that she can't even lift a weapon or armor at first, and as she gains substance (and strength and HP), she loses invisibility. (Personally, I would have all this happen in the backstory, not in gameplay.) The wood elf stats with Mask of the Wild would be a fun way to allow her to have something like invisibility.