How to revert Pixel phone to earlier security level

So after installing the March 2019 security update on my wife's phone - Pixel 1 running 9.0 - it started acting up with:

• Process system not responding
• System UI not responding
• Hanging in middle of viewing/changing settings
• Hanging when a phone call came in
• etc, etc, etc

The update was the only recent change on the phone and the problems were very intermittent.  It would work okay for a while then boom! it would fail.

Tried:

Numerous reboots did nothing - when it would act up, I had to do a force reboot to even getting it working (power button for 20 seconds)

Reset App preferences - nope

Factory Data Reset - nope

Next step - revert back to the Feb 2019 security update level to see if it goes back to being a very reliable phone.  Had no trouble with this phone for over a year (purchased used from Swappa Jan 2018).

Before going any further:

• Make sure you have enabled Backup of the phone to your Google Drive (Settings -> System -> Backup).
• Make sure the backups are current, including pictures uploaded to Photos.
• As an extra precaution, I used SMS Backup to do a separate backup of SMS messages and the call log.  This is essential if you need to setup a different phone as restoring a phone from another phone via the cloud will only work if the target phone is running the same or newer version of the OS software.
• Make sure you do the above when the phone is working fine and you don't expect any trouble.  You may not be able to do these things if the phone is hanging up at inopportune times...

There is no way to revert to the prior software level.  You have to install an image on the phone.  Turns out there are 2 ways to do that:

1. OTA - Install a full image that contains the OTA update that you want.  I learned that this is only useful for installing a new version of the software.  If you try to install an earlier version than what's on the phone, you get an error message "update package is older than .... downgrade not allowed".  Makes sense if you think about as this method does not wipe the data off the phone.
2. Factory - Install a full image that sets the phone back to factory condition running a specific version and in the process will wipe the data.  

Both of these methods use the adb debugging tools on your PC to communicate with the phone.  You need to know how to enable Developer options, USB Debugging, OEM Unlocking, and set the path on your PC.  Be sure to install the latest version of the adb tool.  Don't go down this road unless you understand the above...

Anyway, following the instructions on the Factory images page worked like a charm and ran pretty quickly. 

Important:
So - unlocking the bootloader and locking the bootloader both will wipe the phone.  If you wish to relock the bootloader after installing the factory image - do not restore all your data and apps!  It's a waste of time since you will have to redo it after locking it again.

During the phone setup, you can restore the phone to the way it was before by restoring everything from the 'cloud' - hopefully you have it backed up as mentioned above!

The phone restoration from the cloud now (as of early 2019) works pretty darn good.  The home screen layout and icons were restored, the home screen wallpaper was restored.

Another Important issue:
After rebooting and restoring everything, I noticed that the March update had been downloaded and applied!  Without ever going to the Update setting and clicking on the Download and Install button!  The last 2 hours working on this wasted!!!  This was caused by the Automatic System Update option being enabled by default (hidden in the Developer Options section).

Okay the sequence to follow is:

1. Backup everything
2. Enable Developer options
3. Enable USB debugging
4. Enable OEM unlocking
5. fastboot
6. unlock bootloader - this does a wipe
7. flash-all
8. Reboot but do not go thru any setup or restore, just get it running and enable USB debugging again
9. fastboot
10. lock bootloader - this does a wipe
11. Reboot and this 2nd time setup with your account and restore data/apps.  
12. Enable developer options to access the following settings.
13. Disable Automatic System Updates to prevent it from applying the next security update until you are ready.  Settings -> System -> Advanced -> Developer Options
14. Disable OEM Unlocking
15. Disable USB Debugging if you wish.  

Enjoy!