Sorry, Raven, but the theory that the name "Rhett" may have sprung from the Welsh surname "Rhys" is entirely plausible. Even had Margaret Mitchell invented the name, the author was familiar enough with the surnames of Great Britain to have selected the Irish surname of "Scarlett" for her heroine. (Which sounds a lot better than the author's original consideration of "Pansy O'Hara".)
The following is a quote from Loreto Todd, Reader in International English at the University of Leeds, and worldwide lecturer:
"Reece, Rees, Rhett, Rhys -- these names seem to be derived from the Welsh 'rhys' and to mean 'ardour, passion'. The first two forms are anglicisations of Rhys and Rhett is probably a pet form of the name... It is, of course, possible that Margaret Mitchell made up the name, but the use of 'Rh' suggests a Welsh link."
In addition, considering the meaning of the name "Rhys", one can see that this would be an entirely fitting name (in whatever form) for the character of Rhett Butler.
Margaret Mitchell carefully crafted her novel *Gone with the Wind*. I don't believe that she would have named her characters (along with some input from her editor, Harold Latham) without these names having a significance of some sort. Even the title of her novel was significant in having come from a line in Ernest Dowson's poem "Cynara".