"Saying, by way of introduction, "this is the tune of a song sung when milking a cow" is mediation"

I'd have called that 'providing context'. As soon as you call it 'mediation' you throw suspicion on it, for reasons discussed earlier.

Steve's example of Cecilia Costello's 'Grey Cock' is a fine case of something, but I'm not sure what. At what level in the chain might that collation have occurred? Conscious rewrite, individual creativity, 'folk process'? Queen Caroline Hughes presents an example of a singer mixing elements from different ballads, but her outcomes were nothing like as coherent as 'Grey Cock', and she was a bit of a one-off anyway.