Will,
"Jim, can I just dig a little deeper and….."
Sorry about the long pause, but I found your questions took a little time to think about - truth to tell,
I don't think I ever rationalised the way we worked, or even if there was a set method.
My first instinct was to say – like we used to about folk song, "I can't define what we did fully but I know what it is when we hear it", but that wouldn't be strictly true.
It needs to be remembered that The Singers Club was very much a 'policy' club, based mainly on presenting traditional song, so anything which was performed there had to fit in with this; that's what we came together for and that's what the audiences expected to hear.
MacColl, with others, (Charles Parker, for one) were entirely hooked on the idea that folksong drew its strength from the vernacular; the idioms and accents of the people who created the songs, and he/they believed that these could form the foundations of a new/continuing (depending whether you accept that the old one was dead) oral tradition.   
In my opinion, most of MacColls best songs were based on his and Parker's recorded actuality recorded for the Radio Ballads and other projects. Shoals of Herring drew directly from recordings of East Anglian fishermen, Sam Larner and Ronnie Balls; Freeborn Man, Thirty Foot Trailer, etc., from Minty Smith, Gordon Boswell and Belle Stewart; The Big Hewer and a number of other mining songs from, Jack Elliot etc. I've listened to these fairly extensively and know this to be the case. MacColl was constantly playing or urging us to listen to chunks of these recordings in view to our using them to make new songs and to familiarise ourselves with the different dialects and accents, and above all, the use of language.
The Critics Group was by no means the only ones doing this; I was serving my apprenticeship on the Liverpool Docks and can vividly remember how McGinn's 'Swan Necked Valve' leapt out of the speaker of my Dansette the first time I played 'The Iron Muse' – that was as much about my life as it was the composer's.
MacColl ran several song-writing classes, but there were no hard and fast rules to how the songs were written.
At one time a Singers Club member donated a first edition of Child's first compilation 'English and Scottish Ballads' to be given as a prize for the best new song. The winner was John Pole for his 'Punch and Judy'; hardly recognisable as being in a strictly conventional 'folk' style, but slap in London vernacular.
In order to promote new songs Peggy edited 'The New City Songers' made up from new songs from all over Britain, and later from the US and Australia. It ran into 20-odd editions and made available hundreds of new songs.

This is taking far too long and I don't really want to make it another marathon, so, if nobody objects, I'll take up the rest of your question later.
Jim Carroll