As I said I sing solo usually to the chords of my own guitar and my voice tracks, hopefully on key, with the flattop. I change chords by instinct and do okay but I can't explain to anyone why or when to change except that it " sounds right". I don't read music but can recognize notation enough to help me play a melody that I already know. My voice does not have a lot of range (baritone) so I carefully select the best key for the song. My diction is clear and although I have and know my limits, I have a hell of a lot of fun performing and seem to be able to please an audience.
The main question I suppose deals with how it is possible to make your voice harmonize into a chord structure or triad while listening to the melody. I see no problem with what I, perhaps incorrectly, call harmonic harmony, but wondered about chordal harmony.
In any case my singing will always remain with a simpler structure but I am fascinated by the barbershop style especially. Thanks again to all for your great response to this thread!