Alafair is probably not a form of
Alastair as in my experience it is exclusively a female name. But it certainly has been the name of many real people; it's not just a fictional name or a pen name.
Alafair has been used in the Southern US for well over two centuries. There are 40 women named Alafair in the 1850 US Census, the oldest being born about 1767 in Maryland. As to its origin, it's still a mystery to me. There might be a literary origin, but if so, I haven't discovered it yet.