Define 'indigenous', And demonstrate. & whether or not, you will surely admit that I have cast much doubt on your 'added to Barbara Allen after Child'? It might not have been there [tho can you be certain?] when Pepys heard Mrs Knipp sing it on 2 Jan 1666; or even later. And it might have been there ab origine in those ballads you take to 'qualify', tho who & how's to say? But it was certainly there in most of the 100+ variants of B.Allen which Sharp so famously found in early C20 in Virginia alone; it had by then become a much-loved traditional part of the ballad amongst those who sang and preserved it; and, in US at least, we can surely extrapolate [see Bronson's #159] had been so for at least several generations.