I think the difference between the duchess burns and leslie is the magic prevents the use of healing arts. In leslie case I think it was normaly fire but the reason she was saved was more a location bound ghost type of thing. At least it that is how it seemed to me and after rereading what it out. Plus her shadows act as if they have a will of there own at times, adding to ghost give her more power idea. When it comes to healing leslie I think it is to show a difference between the duchess and her father, show that she wishes to help leslie, not just use her. The matching scar thing between the duchess and leslie would have been an interesting idea, but it feels more like the duchess wants to bring back the innocent child leslie should have been.