There was an instance of the updated version's installer which remained in Task Manager despite the toggle of that "Abort" switch (thanks for that -- the developers might want to correct the oversight), so I terminated it and tried the installer again -- no joy, as the same behavior repeated. So, I again terminated the process tree in Task Manager, then rebooted and tried it again -- no joy, again. So, a complete uninstall was in order.
That older machine still has a registered version of Revo Uninstaller on it, so I launched it with subsequent instruction to root out the CCleaner installation completely under an "agressive" profile; once that ran through the Piriform uninstaller and found remaining dregs, I deleted all of those (one of which was the offending file, ccleaner64.exe -- that sucker was a lot more resistant to removal than I would ever have thought normal for non-malware) and again rebooted. No sign of CCleaner, so I could try a completely fresh install, which seems to have worked so the issue-du-jour can be marked as "resolved".
Does make me wonder about the recent trajectory toward more user-unfriendly behavior on the part of this long-utilized app with the transition to Avast's umbrella, though...