B. Mainly agreeable...but as I said before...the essence seems to be that you have NO "flying hand" at all with "concertinas"
D. I'm afraid you come into a major conflict with the whole "Accordeon World" by this view. For more than 50 years the "Free Bass" accordeon is an established term for *accordeons* with one note per button on the left/bass side, instead of the "Stradella" bass system or other similar chord ones. According ( !! ) to You the Free Bass Accordeon then would be a "concertina"...is this what you mean? You can always test it on some Accordeon chat site and report back...
As to B: The reason why the concertinas have no "flying hand" is that they are too small to strap on, therefore the hands must support the instrument while the fingers press the buttons. And the "non-linear" (i.e. 2-dimensional) button layouts of the concertinas also stem from the small size. The small size, in turn, is dictated by portability. Victorian amateur musicians visiting each other for a musical soirée, or jolly Jack Tars humping their kit aboard ship, wouldn't want to lug a full-size accordeon about. As to the Bandoneon: its square format would also be awkward to strap on, so although it's invariably played on the lap, it retains the concertina-style handstraps. Let's say, it's as big as the "small" bellows instruments can get.
As to D: Free bass, as i understand it, is an enhancement to the traditional accordeon. My Russian friend's bayan is a "convertible" - he can switch from Stradella to free-bass on the left side.
Let's be Darwinistic about this: Genetic variations do occur; some of them prove useful, and catch on, others don't. The free-bass on the accordeon has become established; at least in its "convertible" form, it is produced in series. The Franglo seems to have remained a prototype for which there is not enough demand to warrant series production.
But to go back to terminology: The uninitiated (in any area of expertise) more often than not use terminoligy incorrectly. But if you show any accordeonist (be he diatonic or chromatic) the workings of your concertina (whatever system it may be), he will say, "That's not an accordeon!" Similarly, show a concerinist of any ilk a melodion, CBA or PA, and he will say, "That's not a concertina!"
It's all so simple, really, if we don't get sidetracked by anomalies that can and do occur. The exception proves the rule. Franglo and free-bass accordeon are exceptions to a pretty consistent rule!