SJ Lepak, let us be clear about the fact that it was not just my professor's analysis. He didn't pick it up out of thin air. He found it cited in several different prior sources, but he agreed that the assessment made perfect sense.

Once again, let me point out that whether or not the rose and briar motif was connected with "Barbara Allen" when the ballad came into existence in the first place, it got added in some variants as the ballad moved through the folk process, in the same way that other "floater verses" ("Who will shoe my pretty little foot," for example, as cited above) attached themselves to various other songs and ballads.

That's the nature of folklore and the folk process.

Don Firth