How facinating!
It's possible that as with all new things the 'Ms.' had it's fair share of misconceptions along the way. Or perhaps a wide array of women used it, and some used it as you described Sarah. (Though I've never met a woman who used the abbreviation Ms. for that reason. They are all 20-something though)
If I had time tonight I might google-search the historical beginnings of that word.
Ms was just an attempt to find a female equivalent of Mr. So basically an abbreviation that was consistently the same. Mr. Darcy's name-identity does not change. By the end he is still Mr. Darcy. So the Ms. was born: a name-identity that isn't effected by any male. Or so I remember reading way back in college days.
That said I have to agree: where writing set in the regency era is concerned, it has to be Miss Bennet. I think it was perhaps an unconscious move on the part of the writers you read. Since in modern usage, Ms is common.
