Another couple of thoughts.

1. Any definition of 'folk' in line with the way that the word is currently used would have to include Bert Jansch singing "Needle of Death", the Ukulele Orchestra of Great Britain playing the theme from Shaft, Jim Moray's recording of "All you pretty girls" and Sinead O'Connor singing "She moved through the fair".

2. As if that wasn't hard enough, the same definition would have to exclude Ed Sheeran singing "The A Team", the Love Unlimited Orchestra playing the theme from Shaft, XTC's recording of "All you pretty girls" and Sinead O'Connor singing "Nothing compares 2 U".

I suggest that it can't be done. Folk(1) can be defined - although not without some quite interesting grey areas - but folk(2) is just "whatever people are calling folk at any given time".