In chapter 1 of "Heidi", Dete and her old acquaintance Barbel are leading Heidi up the mountain to meet her grandfather. The author, Johanna Spyri, starts talking about die Dete and die Barbel, for example
Aber die Barbel hätte schon lange gern gewusst, ... .
But sometimes the die is not there:
»Und wenn auch«, sagte Dete trotzig, ... .
The die is included about a dozen times each for Dete and Barbel, and das is used for Heidi once, but more in the next chapter. This was confusing to me at first, but it's covered in SE questions here and here and in a Reddit thread here, and this AdA map. The upshot, as far as Dete and Barbel are concerned, is that in southern Germany it's common to use an article with a first name. (The boundary, which is quite sharp, roughly follows a line from Dresden to Munster. This would seem to include much, if not most, of population of Germany, not to mention Switzerland and Austria, so I'm very surprised this isn't covered more in grammars and German textbooks. Articles other than die also appear in some areas.) I still have a few questions though.
The die is used with Dete and Barbel, but Heidi gets das. I assume that's because Dete and Barbel are both old enough to be called eine Frau, but Heide is still ein Kind or ein Mädchen. Is this correct?
It appears that the article wouldn't be used when addressing the person directly. For example Dete says »Bist du müde, Heidi?« but she would never say »Bist du müde, das Heidi?« Is that correct?
Even in the third person, Spyri uses the article sometimes and sometimes not. Is there any meaning or connotation attached to it? One of the answers talks about it being an "emphasis marker", but I'm skeptical since it doesn't seem to match the way it's used in the book.
I can understand how Spyri, being Swiss, would prefer to use southern speech patterns, but I'm surprised that her publisher (German) would allow it since it's not Standard German. (There was some debate in one of the SE threads about what Standard German is or should be, but I would prefer to leave that can of worms unopened. Let's just say it's the German they teach to foreigners.) J. K. Rowling can give Hagrid a regional accent, but I think her publisher would object if she tried to write an entire book in that accent. So is it common for an author's regionalisms to appear in a nationally published book like this? Or is Spyri trying to give the narration a more homey, less formal feel by including the occasional southern expression? Other than that, much of the narration is rather florid (imo), even by 19th century standards, and especially for a children's book.