"So what does this prove"
Doesn't prove a thing - some people pick up their accents at home, others in the street - simple as that.
Making a rule of who does what is either stupid or (as in this case) agenda-driven.
MacColl was reported in D G Bridson's book, 'Propero and Ariel', to have been found singing busking for pennies from a cinema queue in a broad Scots accent and in Scots Gallic in 1931 - before his Theatre Workshop days and before his career i folk song was even on the horizon - and certainly before he deserted from the army.
He was described as singing "Scots songs, Border Ballads, folk songs and Gaelic songs he had learned from his mother".
From conversations I witnessed between Ewan and Betsy, he slipped from ne accent to another naturally and with ease.
I tend to do the same - I left home with a broad Liverpool accent, within a month that had become tinged with a Mancunian one, I never picked up much of a London one in 30 years, but it did become tingeed with Irishisms because of the company I kept, which deepened when we moved here, but nowadays, my partner Pat always knows when I am talking to my sisters on the phone because I revert to my original accent - broad Liverpool - mone of this is self conscious or deliberate and it certainly isn't because I'm on the run from the army!
By the way guest, "It is a completely different thing to enlist then desert" - MacColl didn't "enlist" - he was conscripted in July 1940 and had deserted by October.
Jim Carroll