"So in answer, a folk song is anything an informed person says it is."

That, of course, is not, by any stretch of the imagination, a definition. It is pure wish fulfilment and an infinitely wide net to catch any music you like so that you can attach the label 'folk music' to it. An infinitely wide net with an infinitely wide 'logical hole' in it - perhaps you shouldn't be preaching to other people about 'logical holes', glueman!