Brian
Regarding who started or continued the process of prioritising the Child Ballads and indeed the triple section layout, it looks from what I can see is Sharp and Campbell started it off. The next I have is the great Louise Pound (American Songs and Ballads) who probably started off the tripling. (Child ballads first in number order, then other British ballads, then native American ballads). Then comes Cox (Folk Songs of the South) in 1924 emulating Pound, Mackenzie in 1928, and Davis in 1929.
I haven't got any of the Barry/Eckstorm Maine books, 1927 & 29, but I bet they have the same system.