Jim writes -
I am referring to the natural unexaggerated and often understated speech of our folk songs.
So is that the factor that makes them unique?
Are there no written songs that show these qualities?
Are there other factors and facts that appear in folk songs and nowhere else? I mean something quantifiable.

If you can provide a list of factors that makes folk songs identifiable from all other genres of song then you are really on to something - but we need the details.

You write
On the contrary, they are creations of the "common" people rather that of music created by commerce for profit - that is what makes them unique.
whilst also writing
If you want firm evidence there isn't a shred of it either way, so all the modern scientific methods have nothing to work on apart from tracing first printed versions unless you have any way of showing these to be the first, first printing means nothing whatever.

If you think that there isn't a shred of evidence about the origin of the songs, are you not contradicting yourself to claim that they are the creations of the "common people". I don't think that you can have it both ways.

Are the undefined "common" people who may or may not have created these songs incapable of thinking that they might make a little honest money by singing them to a broadside publisher?