How does one who has sung soprano in church choirs most of adult life, lose that formal almost polished sound?

The 'folk sound', for me, is the sound of the speaking voice - if you can't hear your speaking voice within the notes you're singing, it's probably too polished.

McGrath: if you've got a Virginia accent, sing the songs in a Virginia accent

Seconded. Affecting an accent will just give you one more thing to worry about. Keeping up an accent is like painting a wall, you need to make sure every word is covered - the best accent in the world will sound terrible if you miss a couple of syllables.

The other thing about accents is that they're complicated. When I hear people affecting an 'American' singing voice (which happens all too often, even on echt English material) I always wonder where that particular accent would actually be from (Florida? Oregon? Scranton, PA?) In your shoes I'd be worried about ending up with an equally non-specific 'English' singing accent.