"But as for the question can we date the instrument imagery to before 1794,"
Musical instruments as symbols go back as far as ancient Greece - at least.
It should be a mere formality to link them to poetry and song.
As far as 'Rose' is concerned, it seems you have answered your own question with your Doc Watson song - simple accidental transmission by somebody along the way.
It certainly doesn't occur in any of the versions of the poem/song in any of the collections we have to hand.
Jim Carroll