And a final followup followup...
The error noted in the original post Subject is long gone so I will be marking this problem "solved" even though nothing at all was solved, I simply threw out the old and got a new OS (and that for sure is not a "solution"!).
To end this particular set of problems I decided to create a Windows 10 Install DVD and use that for the next attempt at reset. The creation of that DVD was straight-forward so no more about that. When I ran the install off that DVD I chose to "install" over, not "repair", the current OS. The installation went smoothly and I chose to use a local user, not a Microsoft Account user, to finalize whatever needed finalizing.
When I got to the point of the installation where Windows says "Let's get started" (or something like that) it displayed a normal logged-in screen with the "John Doe" logged in AND the Store icon actually appearing in the tray where it belongs (Yay!). Checking out the event logs showed numerous error entries related mostly to apps that I've simply chosen to ignore; I assume most are due to the newness of the installation and will clear up (somewhat) after the installation has time to self-update.
One problem with the install was an incorrect timezone (Pacific) being defaulted to rather than my Eastern - there was no installation opportunity to change that timezone so I did it after all was said and done. One side effect of this were events in the event logs having bad timestamps and, with the Admin ones at least, thus being sorted incorrectly when displayed.
Anyway, considerable putzing around suggests this most-recent "reset" has created a healthier instantiation of the OS than has been on this laptop for quite some time. I tried out a few store apps, including installing the Reader app, and all worked as they should. I recreated one of the other two users of this laptop and restored all her documents and downloads from the backup expansion drive - that went smoothly and she is able to log in, etc. However, the event logs filled up with a gajillion app errors once again - not sure why (nor care at this point) but assume they will clear up eventually - maybe after a few more logins by that user. I have one more user to recreate and will do so as with the prior two - no Microsoft Account stuff at all (in fact, I have zero clue what the Microsoft Account stuff buys us; I believe none of us have ever used same for anything overt).
Oh...one anomaly...the "install" saves the old Windows in Windows.old. That old Windows had, of course, saved the PRIOR older Windows, too, during the broken first attempt at an in-place reset, so there were two files, one "Windows.old" the other "Windows.old.0". I was able to remove "Windows.old.0" (right click delete) but got an error while trying to remove "Windows.old" saying I had to get System's permission to do that. Right now I'm simply not willing to figure out how to do that so am just going to leave that large junk folder sitting there...