Spotlight indexing - Privacy list error - Mojave NOT indexing files, email, apps, etc (local files)

Solved! After reading dozens of other related threads and learning all of the terminal commands, safe mode techniques, i.e. tinkering, I concluded that *this* problem was not solvable via tweaks and that I had to do the big fix: re-install the OS. I had downgraded from Catalina due to problems with an older fully paid copy of Adobe Lightroom as well as many old apps that were not 64 BIT apps (comparability mode?) so needed to install Mojave which I describe below.

This sounds painful but it wasn't. The longest step was backing up my drive. I am very happy with the outcome: Everything looks just like it did prior to the wipe EXCEPT Spotlight is now working for local files & apps and I can search my mail - Finally!

Here's what I did:

1. Time Machine: backed up all my user directory files; applications etc. to an external 500MB SSD drive
2. Created a bootable USB install drive with a handy (free) application called "Disk Drill" and a downloaded copy of the Mojave Installer from the App store.
3. Booted the computer from the USB drive (Restart + Option Key)
4. Disk Utility: Deleted the two volumes (Mac HD and MAC HD Data) created by Catalina.
5. Disk Utility: to create a new APFS volume
6. Installer: Installed a fresh copy of Mojave
7. File recovery: My final step (after reboot) was to recover my file from my SSD backup drive using Time Machine as part of the Mojave initial setup process. All my settings, application registrations, etc were preserved as if nothing had changed. Sweet!

Good luck!