Jim,
Regarding usage of the word 'folk' Don has already answered for me. Like him I'm not suggesting we drop the word, just recognise that it has another wider meaning today whether we like it or not. That wider meaning actually encompasses your meaning. I'm not suggesting we alter any titles from the past that include the word, although I do think if these organisations/writers were setting out afresh they might think twice about wording their titles in this way. As for the EFDSS their activities frequently associate with and include the wider usage of the word. Also I don't see many more recent organisations using the word in their titles, particularly in Ireland, ITMA for instance. Those using the word 'folk' anew are the organisations that encompass the wider meaning.

Now, coming back to the OP if we may, I would therefore, if we can agree that we don't need to have hard and fast boundaries, for the purposes of MUDCAT threads and related can I suggest that 'traditional song' be defined tentatively as any song that has undergone the oral/aural process. If we can agree on this temporarily then we can start to add in other qualifiers if that is the general concensus. If the majority want to exclude certain genres or throw in timescales I'm happy to go along with what the majority decide.
I personally will recognise that other traditions have played a large part in what we generally perceive as 'traditional song'.