"I don't believe there can ever be such a thing"
As what exactly?
Folksong is far for than repetition, which appears to be the point you were making - it's a folk song because people sing it and change it in the process - bit facile, doncha think?
The fact that things get changed don't make them "folk" - far more complicated than that
Folksongs are those used and probably made by the folk as a form of self-expression - they are, or were songs either taken into the communities from outside and adapted to fit the needs of those communities, or actually made within those communities and absorbed into them, usually losing the trace of the original maker.
Take a look at the Clare site - the number of songs that deal with the subjects that deal with community or national events, such as the cattle-drive songs or those describing what happened to those forced to emigrate ('Sons of Granuale or Seven Irishmen, for instance)
A million miles away from Boomtown Rats' somewhat meaningles and largely repetitious take on Columbine.   
Sorry - didn't understand your somewhat convoluted bit on Mozart
Folk song refers to origin and ownership - try telling Geldof et al that his song isfolk, ours, in the public domain, whatever and see how far go get with him or his legal team.
Not only is it not a folk song, but our legal system makes damn sure that it will never become one and will always belong to who wrote it, and in order for us to perform it publicly, we would have to pay for the privilege.
Lumping anything the Music Machine produces in with the 'Voice of the People' (widely used phrase - not mine) debases the coin and does what people have been trying to do for centuries - disenfranchising the people from their culture.
Hasn't this somewhat reactionary attitude already done enough damage by throwing open the 'folk song' clubs to the predatory PRS and IMRO jackals who are more than happy that we should pay for the privilege of singing our folk songs in our folk clubs.
I've stated my personal likes and dislikes, but in the end, this has nothing whatever to do with the fact that it has been recognised for over a century that these songs came from a different stable as the mass produced ones.
Any evidence to the contrary would certainly be viewed with interest, as far as I'm concerned.   
Jim Carroll