Michael - I was curious about the use of "folksong" in the book.

As for the actual origins of the term, the OED is quite interesting on this one. It has an 1870 usage in the title of a book by a Lancashire antiquarian: "Folk Song and Folk Speech of Lancashire: on the ballads and songs of the County Palatine". After 1870 there are quite a few occurrences, but before then there's only one - and that one seems to be a direct translation from the German Volkslied: "The Three Little Roses. A German Folk's song." That's dated 1847, so well after Austen's period.

McGrath - "a cool hundred", "a cool thousand" etc date right back to the eighteenth century; it's a separate form of "cool" from "cool kid", "cool Daddy" etc (which date from around the time of WW1). (Joe Gargery could have called someone a "cool customer", but that's a different meaning again.)