"More or less authentic" in the sense that they're sensitive interpretations of genuinely collected texts and tunes, even if the texts are sometimes conflated (as I recall) and the tunes supplied to tuneless from similar versions. MacColl & Seeger conscientiously stay as close to traditional style and spirit as professional performers can.

MacColl's rendition of "Lamkin" is one of the finest a cappella ballad performances I've ever heard.

As for dying traditions: while I certainly agree with you there, my guess is that if somone had gone about collecting like Sharp in, say, the eighteenth century, he would have encountered plenty of mangled, half-forgotten, and partly ad-libbed texts and tunes. Of course, he would have found many of the better kind as well (as have modern collectors).

"Blood and Roses" was a different sort of project, of which I also think highly.