Very interesting analysis. Seems to me the accordion was initially developed to perform "lighter" music with both a melodic element (usually treble) and harmonic and rhythmic (usually bass/chord) accompaniment.
The "three chord" song is a common example of the harmonic function of stradella bass/chords in relation to a given melody.
However, there is a fascinating and very significant link between these two most fundamental arrangements of notes:
the chromatic 12 note scale and the circle of fifths. The arrangement of notes in a circle of fifths has been of great importance for centuries now, but here's a short video about the mathematics of why 5 is the magic number...
I take your point here... but maybe I would call them "developments" instead.
To me, having a free bass accordion with 3 or more octaves of single notes on the left side (usually arranged chromatically/minor 3rds or in fifths), allows the musician more polyphonic options than the single octave of stradella bass/chords. It lets the accordionist explore keyboard music beyond the usual stradella bass repertoire.
To me the difference between the stradella bass accordion and the free bass instrument is that the stradella accordion is a very versatile musical instrument, whereas the free bass accordion (converter or MIII etc) is "pound for pound" the most complete musical instrument in the world.