Folks, I get ready to remove the entire system user where the corrupted mail is then want to try one more thing. Keep in mind that Mail for the other two users has no problems.
I have re-read this entire thread and realize I have not been, methodically, following your instructions; reporting to you what I've done.
Besides getting the Combo Updater downloaded, I have followed instructions, "Remove Mail saved state" with no result.
Here is what is happening when I open Mail in this users folder:
1. Usually, clicking any Mail icon starts the spinning wheel. (Force Quit will not quit mail. Shutting down and re-booting doesn't quit Mail. To force quit I have learned to go to a browser> apple> Force Quit.)
2. Once in a while preferences has opened and I can get as far as to click Accounts. If it opens at all, it is completely blank and spinning wheel starts.
I found this page in Apple Support: Spotlight: How to re-index folders or volumes - Learn how to re-index a folder or an entire volume for Spotlight.
https://support.apple.com/en-us/HT201716. I don't know what else it might have done, but it didn't affect Mail.
I believe I have done everything I know to do to save the contents of this users folder in preparation for removing it, including having Time Machine running since May 29. What do you think, shall I go for it?
And, I do want to report that HD available storage space is 73.38 GB of 159.18. (It wasn't the only thing, but something my young grandson had downloaded made a lot of difference.)