Nice presentation, Chuck. Some comments:
AFAIK, the term "four quadrant" doesn't refer to the ability to
control all four legs independently (though that is required for
four quadrant control), but to the ability to control all four
quadrants of the torque vs speed graph.
All your discussion of braking talks about using a short circuit.
Braking using a short circuit dissipates energy using (only) the
resistive losses of the motor, which might not be sufficient with
some motors depending on the application. Regenerative braking by
dumping energy back into the supply can be used to get more powerful
braking - or did I miss where you discuss this? Regen using MOSFETS
is cute, as when "on" they can sustain a considerable reverse current,
meaning you don't need such powerful flywheel diodes and can avoid
the diode losses.