My theory at the moment is that Ardith is a respelling of Ardath influenced by Edith. Ardath is a place name (a field near Babylon)in the apocryphal book of Second Esdras. It was used as the title of a novel by the English writer Marie Corelli back in 1889. Though most people have completely forgotten Marie Corelli today, she was a bestseller back in the late 19th and early 20th century and her books were in print for many years. In Corelli's book, Ardath is also the place name near Babylon. But the fact that she used it for the book's title and that it sounded like Edith seems to have inspired a few parents to give the name to girls. It pops up on the SSA top 1000 list for girls in 1905. I think what then happened is that people heard of the few girls named Ardath, didn't know how the name was spelled, and assumed it must have the same ending as Edith. The spelling Ardith first turns up on the SSA list in 1913.
That's my theory of Ardith, any way. No idea on Maryth so far. :)