On the whole, I like simple, but I'm glad it's not too simple 'cos if it had been I'd have looked like an idiot for not understanding it in the first place.
I'll hang on to the isolation aspect for now as all your examples are, in different ways, closed communities.
I'll drop the "original composition." That was a misunderstanding of "oral creation." I assumed it meant "initial creation within the community." It seems that you were referring to the deliberate or unconscious development of a song, tale or tune, putting the community's stamp on it, regardless of its origin. Not to be confused with changing the song as a result of Mondegreens or memory failure.
So, my second example might have a "yes" answer if, for example, the singer changes a line to "hush-a-bye, my baby."
The answer to my first question would still be "no," even if the singer managed by chance to replicate the exact intonation of "poor Will Huggins" because the world of poor Will no longer exists. The song would still be a folk song, but not (if learnt from "Songs and Ballads of the West") part of the oral tradition. n'est-ce pas?