I can't contribute to scholarship here, but if anybody wants to have a good time, you can find "Sweet Willie and Lady Margot" in the Ballad Book of John Jacob Niles. That is available for free at Google Books. The song is on page 157.

It has a great tune and good story, and yes, a rose and a briar appear at the end.

I'd like to add that I have been fighting a briar in my front yard since 1977. I've beaten it down to the point where all it does it send up an occasional shoot, yet it is obviously a tenacious plant which is ready to take over the world. Every millimeter of the stem sprouts thin, very sharp thorns. The thorns of a rose are nothing compared to the thorns of a briar.