Brian,
First of all, a big "thank you"! Your instructions were excellent. FWIW, the only point where my situation diverged from your description was at your step #4. At some point during my efforts, I looked for MC20 in the Applications folder, but the MC20 icon that had previously been shown in the Applications folder for version .94 was no longer there. Perhaps it was removed as a result of my having run---albeit unsuccessfully---the MC20 v.126 update process, or maybe at some point in the course of my confused effort to troubleshoot the failed update/install I dragged the v.94 icon from Applications to the Trash, I can't remember.
Anyway, your instructions were successful, and as you said, the process is straight forward once you know it. I'm inclined to point an accusatory finger at Apple here, though, because their app install instructions, at support.apple.com, are quite misleading. Here they are, in their entirety:
"To install apps downloaded from the Internet, double-click the disk image or package file (looks like an open box). If the provided installer doesn’t open automatically, open it, then follow the onscreen instructions."
"Onscreen instructions"? There are none! Yeah, in retrospect I can see that the arrow graphic pointing (if I recall correctly) to the Applications folder represents a prompt to drag the app icon to the Applications folder icon, but that's not my idea of "onscreen instructions". Consequently, I did the intuitive thing (for a Windows user, at least), and simply double-clicked the MC20 app icon that was displayed above the Applications folder in the little window that you referred to, which, as you've indicated, merely ran the MC20 app, but had me thinking that I'd installed the thing. Nope! OK, mini-rant over. :^)
Thanks again for your excellent, detailed instructions and helpful comments!
George