Yes, she is terribly thin, but still graceful and elegant - like Melania Trump.
Princess Kate has had cancer treatment so who knows what is going on.
Looks like she is wearing a wig as well.
No, that's her hair. I follow her pix frequently. She lightened her hair during their summer vacation and it has yet to grow back to her natural auburn brown. She often has a brown rinse or root touch-ups in the winter months to cover any gray hairs, as she is in her early 40s.