THANK YOU SO MUCH
Just a note to those of you who use a Mac as their primary computer:
If you can get access to a Windows PC at work or elsewhere, install iTunes if not already there. Then download the the free 7-day trial of iBackupBot. Start iTunes and make sure you select in Preferences not to autosync devices. (if you try to sync your iPhone, you will loose all your data, or have it replaced with whatever's in the PC's iTunes library.) Connect your iPhone and when it shows up in the left pane, right-click on it. From the pop-up menu, select Backup. When complete, you can then open the backup file from iBackupBot to remove the unwanted email address as above. When you reload the modified backup file from iBackupBot (
not iTunes), the iPhone will reboot and all you data will be preserved except for the deleted autocomplete entries!
(This whole procedure had me really worried because normally when you "Restore from Backup" in iTunes, you lose all of your media and other data.)
It stinks that you have to find a PC in order to do this, but it does work.