Hello everyone, I have good news!
After a few exchanges over in the Behringer forum, a few kind people did some testing in parallel with my efforts, and the combination led to a very satisfactory answer:
The root cause of my specific problem was in the Setup screen of my X32 interface. The "MIDI Control Interface / Card MIDI" box was not checked. Even though all the other Mackie Control functions worked without the box checked, none of the MIDI notes that I assigned/configured on buttons would be sent unless that box is checked.
The Mackie Control protocol in SONAR uses MIDI note G#5 (80) to select the bus (busses) and MIDI note E5 (76) to select the track(s). In the Assign section of the X32 I chose set-B and picked buttons 5,6,7 and 8 to be Mackie Control Bus, Track, Main, and Bus (yes, twice) respectively.
Many thanks to
azlow3 for your offers to code stuff for me, but I won't be needing that help; perhaps keep a rain-check for another heinous problem.
Going forward, the plan in SONAR is to standardize 16 stereo busses across projects. In that way, an arbitrary number of tracks can be routed through those busses, and my choice of doing my traditional ITB (inside-the-box) mixing or experimental OTB (outside-the-box) mixing can be accomplished with simple scene swap in the X32.
I hope others who needed this function can benefit from what I learned along the way.
Cheers!