Lighter wrote: But suppose I'd learned it from a live performance, determined the correct lyrics from a book, and taught it to somebody else twenty years later with minor changes.

In my book, your version would not be traditional. The version of the person who learned it from you might well be, though. If they'd just absorbed it from you, rather than you saying, "Here, learn this!"
above quote from Robert
Robert
I understand your point about oral transmission, and agree up to a point, dependent upon whether the song was traditional in the first place