Easy, Malcolm, easy. It's only a song, and whether Reynardine is an outlaw or a warlock is not that important.
To answer your q., I got the "falling into a coffin business" from the singing of the man I am to accompany. He says he learned it from Pentangle. At least, that's what he said last week. Next week he might say he learned it from an Appalachian Sufi.
Whether it's 20th-century or not (hard to prove) it's a good song and we're doing it.