A very fair review, Phil.
I read Fakesong not long after it appeared. Harker's passion initially impressed me. But his polemics undermine the valuable biographical and textual information he's gathered.
What Harker shows should (but didn't always) go without saying: that folksong texts and collections, like almost everything else, are complicated artifacts not to be taken simply at face value. Fair enough. But what he thinks he shows - that a pure proletarian art was vitiated by self-seekers and frauds - is rather different. So is the idea, as Phil says, that "traditional song" doesn't exist. Harker seems to confuse conceptual categorization (lumping similar things together) with difficulty of definition (saying just why they should be lumped together). By that standard, poetry clearly does not exist; in fact, few things do.
Harker's relentless attacks on long-dead enthusiasts, his dismissal of all their efforts as bad faith and (even in the case of great talents like Walter Scott) bad art, and his tendentious sorting of the world into simple class-based camps of good and evil, make Fakesong - even aside from its alleged fudging and factual errors - a rather unpleasant and less than satisfying read. However, it is worth looking at - with appropriate caution - for the light it sheds on collectors and interpreters.