"I do not think a definition of "folk music and song" is possible"
Too late - it's been done and written up
If it hadn't been possible we wouldn't be here talking to each other
You don't really need a set definition - you can tell by the sound of the songs that they are something special

"Its common usage that defines the meaning of words."
There is no common usage Al - we never won enough hears and minds for there to be one
There goes that Brexit-like populism again
These changes have been brought about not by "common usage" but by a bunch of ageing folikies who neither understand folk song or care about it
Bye-bye 'The Voice of the People' which will remain the Elephant in the Room as long as the present diminishing revival is around
No one on you have come up with what this so-called "common usage" has re-defined folk music to mean - any takers
Won't hold my breath - too wheezy nowadays
Jim