Wow. Do I feel dumb! I just "fixed it". (egg on face)
I had decided to follow the suggestion of
@audiohobbit and try loading the 1.98 firmware (even though I'm happy with 1.81). So I placed the SVS file on the microSD card and went over to the A16 and inserted it. Then I powered the unit on, preparing for the special power off/on sequence to activate the firmware upgrade logic at boot time.
I was sitting right next to the A16 as it went through its sequence of boot-time screens, and finally got to the main menu. I always look at the USER A and USER B lines at the top, to see when they're both finished loading and active. I'm familiar with the WAIT, LOADING, ACTIVE sequence that is normal. Well strangely, this time the USER A (i.e. me) line never presented "loading". USER B finished fine, but not USER A. Apparently I hadn't noticed this yesterday, because the screen is so small and the type font even smaller.
Anyway, what was appearing in the USER A status box was the word "reload". Hmmm... Since the whole menu is still functional at this point even though the speaker map is not yet presented, I pushed ENTER on the PRESET item, and it presented my PRESET 7 which happens to be the last preset I was fooling with yesterday when the red screen and "stuck menu" disaster struck. In fact I was playing with presets 7, 8 and 9 because I was rearranging cabling due to the recent acquisition of the Shield. And I was setting up different Atmos and PCM rooms for these three presets.
So the status of this preset 7 showed as "reload" (which is actually how it normally always looks when you come to it) but in my excitement I forgot that and thought it was telling me that the memory version of the preset needed to be reloaded (perhaps having gotten corrupted). So I pushed ENTER, and the preset DID reload from its stored copy, and when it was finished then presented HDMI (which again my excited brain thought was now the correct state it should be in).
And now, magically, as if by magic, suddenly the speaker map appeared all on its own! In other words, the stuck boot process had apparently gotten hung up struggling to load the current preset for USER A, and had run into a problem. And apparently it was preset 7 (the last preset I was working on) which must have been in some transitory state or something. Whatever the cause the normal load memory from storage for that preset had failed.
But when I manually requested the very same reloading of that preset myself, apparently that worked! And that also overcame the obstacle the boot process was waiting on. And now the stalled boot process retook control and proceeded forward through its final steps, completing normally with the speaker map now appearing (for preset 7 naturally).
And now all is well again. Obviously the 1.81 firmware upgrade retry I did yesterday didn't fix anything because the problem was a corrupted version of preset 7 in some middling state, which somehow couldn't be overwritten by boot-time loading of the preset from storage. But somehow, my manual request to RELOAD the preset from presumably the very same storage worked!! Strange clues.
So, I feel like an idiot. I'm sure I WROTE out (i.e. saved) preset 7 when I was finished making the changes. But maybe I did something else extraneous?? Can't recall. Regardless I am naturally relieved it's not actually a hardware issue after all, as I think we all struggled to see how I was last doing when the red screens occurred could possibly have triggered hardware damage.
Back in business! Yes, I will probably still go ahead with accepting the 4x2 HDMI switch I ordered just last night (when I thought I was going to be using my A8 for a few months).
Case closed, I guess.