I find the subject of 'adopted' accents a somewhat problematical one, both in singing and storytelling.
Our excursion into the storytelling revival left us with the distinct impression of 'poor actors' trying very hard - and failing miserably.
It seems to me that the best way to make a song/story work 'convincingly' is to sing/tell it in your own natural voice with your own natural accent.
I recall the hours of pleasure I have got from listening to the magnificent Alec Stewart, live and on recordings, telling his 'Jack Tales', using his own, flat, laconic delivery.
For me that 'naturalness' is an indication that the song/story is working for the singer/storyteller, which is the greatest part of the battle in making it work for the audience. It is something I find almost universal in recordings of 'source singers', even when their physical abilities may have reduced, It is a quality quite often missing in revival performers.
It is possible, with a great deal of practice, to master 'foreign' accents, but I find it very hard to think of en example where that would be necessary in the singing of folk songs - music hall maybe, but that's different.
Jim Carroll