It might be Bob on the plectrum banjo, Rick. Sounds like it gets a bit fancier towards the end leading me to believe it's Dave. No matter in the end.
In the final chorus, "the Lord loves a sinner, the Lord loves a sinner man"; it's that second refrain of the statement. I think Dave strikes a slightly lower note and Nick a slightly higher. This harmony occurs only the one time, but it drives me through the roof - man it's special and so typical of why the original Trio set the bar for everyone else. Again, put on your headphones and screw the volume up to where it would normally reach the threshold of pain for your wife. It's there and you'll hear it. Just an amazing chord.

Has anyone dissected the chords for this Guard version of the spiritual?

I would drop one more conviction of my own. While I enjoyed and even loved the Stewart Trio sound, "'Round about the mountain" highlights the sound difference between arrangers and choices of music. I don't believe the Trio ever sounded as rich, original, or touched excellence as they did when Dave Guard was when their "acknowledged leader".