Paul, I suspect others have answered your question. All of my collecting was recorded in an area where the singing of these songs was already long dead, apart from for self amusement, so, whereas I have read Dunn enviously, I have no personal experience of song ownership, in oral tradition. I have seen it on many occasions on the folk scene, however. There are threads on the etiquette of this if you wish to pursue it.

Regarding percentages of deliberate song alteration in oral tradition, this would take a lifetime of research to come anywhere near. However I'm pretty certain that it happened much more than most people perceive.

Regarding Harry, I remember reading that he actually had the broadsides of many of the songs he sang.

Regarding the sympathetic plants motive, it would make a very interesting study to try to work out how, where and when it migrated from one ballad to another. Difficult but not totally impossible.