I know SM is set in north Louisiana (in a fictional parish). I live in Louisiana, and I do know of a girl named Melaine, but she's the only one I've heard of. In fact, the only name on the movie that I've ever heard in Louisiana regularly is
Shelby. I've lived here all of my life, and there are no M'Lynn's, Claree's, or Annelle's that I know of.
The accents in the movie are also pretty terrible! I still like the movie, and I cry every time I see.
In reference to Welles' novel, I don't know any Siddalee's either!