"I have memories of trying to sing 'Fathom the Bowl' a tad quicker and with a bit more 'swing' than everybody else. "
It's like trying to run through a pool of water, isn't it?
Peggy Seeger used to get up people's noses by stopping and teaching the chorus - that's why I fell in love with her!
Speaking the last line is a newish Irish affectation - never came across an older singer who did it.
No harm in that - I do it myself regularly - it can help to break the tension for the next song, but I don't think it does much to help the one you're finishing.
One old storyteller/singer we recorded used to finish each song and story by banging his stick on the floor and saying "now"
Fine when we recorded him in the local stone-flagged shop-cum-bar, but didn't do the floor of our Mini much goos - we were never able to record him in his home for various reasons,   
"if you are an unpaid singer "
I would hope you would do this whether you were paid or unpaid Pete - when you sing you are a creative or interpretive artist, not a juke box.
Jim Carroll