I'm from the northeast but I've lived in the south for 5 years, plus my parents now live in FL. I don't know this for a fact, but I've decided the Miss thing is not really Miss (young unmarried miss) but almost like Ms. It's a catchall for a woman. Maybe it's even a simplified pronounciation of Mrs. Mrs. is a strange title anyway, I believe it derives from Mistress but with a weird pronounciation, Missus or Missers. Miss as short of Mistress makes sense, and Mistress is certainly the female head of household, not a young girl.
I think when they say Miss Heather they absolutely do mean a married adult, and that there is no confusion over your status.
And I do know how strange it feels for us Yankees to be called Miss, oh yeah.
Oh, and I think while it was ostensibly nice that you were asked what you wanted to be called, that there was only one right answer in that woman's head: Miss Heather. She was asking you whether it was Miss Heather or Mrs Jones, and if you had said Mrs Jones she would have told her kid to call you that but felt you were putting on airs. (Mrs Jones being okay for older ladies though, or the wife or a doctor or something).
:yeah to the bolded. I'm from the South. Miss FirstName is a catch all regardless of age or martial status. There is very little verbal distinction between Ms, Miss, and Mrs where I'm from. In fact I rarely ever pronounce Mrs. correctly. It's almost always shortened to "Miz". That's true for most of the people around here I think.
That's how I see it too. I don't mind at all, but I'm not easily offended. If parents want to teach their children to address adults with a title, that's fine with me. I'm still undecided on what I'll teach my daughter though.