First of all, Harry is the image of Diana's father as a young man - red hair and all, and secondly, do you really think the world's press have not already obtained DNA samples from Harry and Hewitt and found that they don't match?
She calls herself that because she married Lord Colin Ivar Campbell.
She was born with, among other things fused labia, but is genetically female. She has successfully sued several publications that printed she was a transvestite who had undergone a sex change.
Harry was born before Diana met Hewitt. Di met Hewitt after his military service in Gulf wa c1991 whereas Harry was born 1984. Hewitt was hired to teach the princes to horse ride.