Re duped by four different informants-
If one accepts that Parks encountered fragments of an old gospel or spiritual in the NC-Tenn area, and from the traveling revivalist at Waller, as you say Hays suggested, then only the material from College Station needs re-examination.
A university town is a good place in which to be 'duped.' Like a number of songcatchers, Parks expertise was in another field.
(Not that there is anything wrong with that, Dobie never had an academic doctorate, although he wrote several books which would qualify as research on the doctorate level, and Mody (Moody) Boatwright, who succeeded him as editor of the Texas Folk-Lore Society, was a petroleum geologist).

Oh, well, just more speculation. Keep hunting!