Motherwell's work is available on archive.org.

A number of editions are available, here is a link to an 1827 version dedicated to Sharpe.

https://archive.org/details/minstrelsy00mothgoog/mode/2u

At the risk of being dull, may I point out it isn't strictly accurate to say "all we get from Fakesong is the usual litany of political comment". I won't quote, but page 55 of Fakesong disproves the remark. It discusses Motherwell's focus on collecting from oral tradition. Harker praises this in both Ritson and Motherwell. McCarthy quotes Harker on this (page315).

McCarthy believes that in the early days of putting out the Minstrelsy Motherwell did put together composites, and argues that 'Hindhorn' is one such composite. McCarthy prints a letter sent to Scott asking him if he can help him to 'correct' a so-called flawed version of a song taken down from recitation. This is how McCarthy dates a change in heart in Motherwell, attributing it to the reply from Scott. But I cannot find Harker discussing the 'fake' composite: he is more interested in the fact that Motherwell changed and developed as a collector rather than an editor. Had Harker been focussed merely or even significantly on discovering 'fake songs', he would have missed a trick here.

Whatever one thinks about Harker's brand of left-wing politics, I am not alone in finding it interesting that such a right-wing character should be so interested in old songs, some of which were being sung by people of low social status. McCarthy also touches upon this. He calls Motherwell a 'High Church Tory' and wonders what M's informants made of him.