"Is the work of anthologists and collectors from Percy to Lloyd, in its entirety, a 'systematic process' that can also be applied to all the rest..."

I don't think so either, but I think Harker presents it more as a cumulative narrative about 'the Folk' amongst the middle classes. His lowest level of granularity is the collector (or maybe 'the collection' as in a group of songs bundled together somehow). He is not talking about things at the level of a song so the seperate elements that Steve Gardham listed can be lumped together as 'mediation'.

Things get lumped together when similar features make them stand together within a discussion of a much wider scope. Steve Roud's discussion of Fakesong and The Imagined Village, and the various reviews of either or both, left them grouped together in my mind. Having since read them I think that is misleading (and unfair on G Boyes) if focusing on the detail of what they say but not so much when when thinking about 'the story' that has come down to us.