As with all other Music Idioms (be they Popular, Classical & Sacred) process and fluidity are integral to the nature of the beast

Maybe so, but...

Given the various conditions of the Natural Habitat of what we now think of as being Traditional Folk Song (primarily oral, no copyright, highly creative master singers and song makers well versed in their tradition, static communities interfacing with other static communities via marriages, migrations, mendicants, travellers, no radios, no records other than ear wax &c. &c.)

it seems to me that process and fluidity were a much more prominent feature of w.w.n.t.o.a.b.T.F.S. than they are of art forms that are written down, widely communicated in that written form, broadcast on the radio, recorded, etc; you could even say they were a defining feature. As witness the fact that there are approximately seventy-three different versions of The Unfortunate Rake (or Lass) - and slightly different versions of those versions are being performed at singarounds most weeks - while there's precisely one authentic (ha!) version of Streets of London, of which faithful copies are performed at folk clubs equally frequently.