Well, my new user name and old user name were slightly different, for clarity's sake, and of course the old Mail folders were "owned" by the old user ... but even though I am an admin user, I could not get the permissions to change. They would show the proper user names in the Get Info text fields, but there were still red minus-sign badges on the folder icons that showed the folders were still read-only. Permission changes were not taking effect. Comparing this Mail folder with that of a test user account I created showed that these permissions were definitely wrong.
Finally, I moved the entire Mail folder out of my Library and started Mail again -- now I can finally delete mail from my Inbox.

