This is best described as a variation on grid format, with up to 4 chords per bar shown. G G means the chords on beats 1 & 3 or 1 & 4 according to time signature, GCG are chords on beats 1, 2 and 3 in 4/4 and G - D would be chords on beats 1 & 4 in 4/4. Notes as opposed to chords are shown with a /, so G/B is G major with B in the bass, and /F/E/D would be a bass run with no chord identified.
About Keys and Root Notes.
This is my own idiosyncratic (ie incorrect) presentation. I don't much care whether the tune is mixolydian, dorian or what. All I want to know is the equivalent major key, which gives me the notes on the fingerboard, and the root note of the tune, which is naturally different from the key if its not a major scale.
About Chord Choice.
The way this old school untrained bass player operates is to improvise a bass line working from guitar chords. Well, it works for me, except that these tunes were never written with chords, and I've never really understood how melodeon players pick their bass buttons. There certainly isn't a definitive right or wrong arrangement for these tunes. The tunes are mostly from the Ewell tune books. Some of these are presented with chords, and for others I've found chords on line. But in a good number of cases there 's been nothing to do but sit down with a sequencer and a guitar and work an arrangement out. Its not something I've really often had to do and I'm not very good at it, so some of these arrangements, particularly those marked chords JimC, are probably shaky. Its also possible that my arrangements, created against just the melody, may clash with the arrangements used by melodeon players. They should get refined playing, and hopefully I can remember the refinements and update.