Problem is solved. I went through too many steps with Dell to document here so I will focus on the salient points.

• Soon - less than a half hour - after my initial posting Dell reached out to me; I didn't have to contact them.
• After uninstalling the Dell Support Assist Dell Update Plugin I was able to re-install Dell Update but still not able to re-install SupportAssist.
• I renamed the folder C:\ProgramData\SupportAssist so the next install attempt would create a new folder that I could inspect. In the file C:\ProgramData\SupportAssist\Client\Agent\logs\ApplicationLogs\SupportAssistAgent.txt I found the following lines at the end of the install log. The beginning of each line has been cut off for brevity:

         Installation Failed with return code : 1642
        Upgradation of SupportAssist failed.
        SupportAssist download/installation failed with return code: SupportAssistUpgradeFailed
         Application Exit Code: 19

• I Googled return code 1642 and also with the 'Upgradation of SupportAssist' message I guessed that there must be some remnant of the previous install that was triggering an upgrade attempt instead of a new install. However I did not know what the remnant could be assuming that was the problem.
• Dell eventually asked for a remote session. We did the remote session and it looked like the Dell support team deleted the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Dell\ UpdateService\SupportAssistAgent or simply Dell\SupportAssistAgent. I wasn't able to take a screenshot of the exact key. I did not observe any other changes being made to the system besides the registry key delete.
• My best guess is the registry key that was deleted was the part that was tricking the install into trying an upgrade of an installation that didn't exist and caused the install failure.

In conclusion my Dell Update is working again. So is SupportAssist. I will keep Dell Update running (driver update process is fabulous) but stop the SupportAssist service as it is taking over 650 Mb of memory at the moment. But it is there if I ever need it again. Dell, please fix SupportAssist to not use so much memory!

I would like to recognize Akshatha and Dilip for their valuable assistance.