John,
Regarding Mrs Brown there is a new book out comparing all 5 manuscripts for the first time and looking at her background, which would throw up some interesting insights. I haven't got a copy yet.

Also if you haven't read it yet I strongly recommend Mary Ellen Brown's book 'Child's Unfinished Masterpiece' which includes Child's correspondence with his Scottish collectors and informants and gives some insight into his changing feelings about the authenticity of some of the ballads.

I'm currently transcribing the Peter Buchan Mss in the British Library which are very illuminating. Many of the ballads are straight off stall copies, others heavily tampered with, but there are some great earliest examples of well-known folk songs.