Let me quote Steve Roud from the General Introduction to that book, whose launch I sang at.
"Performers, of course, are under no such obligations as regards fidelity to source, although in the folk world there have often been strong notions of being grounded in 'tradition', which at least implies a degree of authenticity...
"This divergence of expectation between scholars and performers has resulted in a regular irritating misunderstanding of the question of accuracy and alteration in folk music. One argument runs that because folk songs change in transmission and performance it is therefore perfectly acceptable for collectors and editors to alter the songs they are presenting to the public...
"The fact of the matter is that the editors' and performers' need may overlap, but are very different."
You, sadly, have confused the two. From as far back as we know, performers' editions have been arranged. John Gay did it in The Beggars Opera, at a time when folk songs were in their birth, Edward Bunting did it two hundred years ago, Vaughan Williams did it a hundred years ago, and Steeleye Span did it forty years ago when they published their arrangements. When we sang at the Southbank in Tapping the Source, Steve played the recordings first, then we performed the current arrangements, as did Martin Carthy (and yes, I've studied arrangement under him too). The original notes are there, the modern arrangements are in the Archive too now. That was then, this is now, and where will we be in 2113? Look at Lilibulero, for example, as a tune which has wandered over the years. You cannot freeze the music forever, music in the last thousand years has moved from Psalmic monody to Bach's mathematics, to Fauré's Romanticism, to Cage's asonic tonality, to... The historian has to keep the situation at a given date clean, but the performer can move it on, must move it on, will move it on.
Just because you're not certain about the roots doesn't qualify you, therefore, to make the criticisms you do. Heck, the Americans include things like that ghastly pastiche Ashokan Farewell in their "Traditional" repertoire. Pastiche for all the clichés it nicks, the diminished chord of Give Me Your Hand, for example. There's a lot of work in the Northumbrian tradition whiose composition is known. And on and on we can go.
What you must do, on the other hand, is unknit the perversions previous performers have committed, and then, understanding what the original tune probably was and why, add your own. A Tudor tune is going to be very different from a nineteenth century one, for sure. Should we therefore also perform it in a Staffordshire accent? Particularly when we come from elsewhere? It's why if we take these on board, we make them our own. Nor for that matter do we need to stop there. Look in the Take 6 Archive and you'll see Steve picked just a few of the virtually unknown songs the collectors of a hundred years ago gathered. There is a wealth of others there too.
And just as that is one source, I fail to see how you can say "Vaughan Williams Good, Moore Bad" to another. Much of what RVW arranged was equally bad. What you must do is to see what can be made good for now, music being an ephemeral art, we know it will no longer be the same in a hundred years time.
In the first ten tunes in the Index of The National, I find The Keel Row, The Bailiff's Daughter of Islington, and Barbara Allen. Further on, The Barley Mow, most of the Ravenscroft rounds, Almost all the Welsh tradition, and a big chunk of the Scottish are there. And you'd ban the lot! Poppycock, it never pretended to be only a folksong book, it's just you learned a fairly justifiable prejudice in your youth which needs reexamination now, having swung too far the other way.
If you're performing a folk song, threfore, you must muck about with it. Steve himself was quite happy with that. Now OK, if pretending to be a Victorian gammer floats your boat, good luck to you. I'd hope you had some kind of background enabling you to go there. But me, I'm happy with being me and bringing the songs to me.