Well after about 45 minutes with Apple Tech Support, I was able to (re)establish access to the "Mail, Contacts, & Calendars" screen in System Preferences.
Neither the tech specialist nor myself are quite sure how we accomplished it, and I will try to retrace our steps here for those of you *still* experiencing the problem.
FYI this is a "user" problem, not a system problem; that is to say, if you create a new user (in System Preferences) with admin status, you should be able to access this setting under that (new) user account. Thus, we established that there is something funky going on under the user settings.
That being said, see if this works for you:
1. Go to your Library folder (~/Library/) (Open Finder, then click "Go"; hold down 'option' key, and your Library should open)
2. In the Library folder, find your "preferences folder” (~/Library/Preferences/) and copy it to your desktop (click on a file, hit cmd-a, then copy, and paste to desktop)
3. Delete (move to trash) everything in your Library ---> "preferences” (~/Library/Preferences/) folder EXCEPT the following file:
com.apple.mail.plist.lockfile
4. Restart (After restarting your desktop image will be reset, as well as many other options you may have set)
5. Move everything back from the "preferences" folder on your desktop BACK to the Library ---> Preferences folder (remember: to access your Library folder, go to Finder, click on "Go" and hold down "option" key, and it will appear)
6. Restart
7. Open "System Preferences" and click on "Mail, Contacts, and Calendars" and you should be all set
You can now just delete the "preferences" folder from your desktop (only after being sure you've copied everything back to the Library ---> "preferences" folder; you can also empty the trash at this point, assuming all is working).
NOTE: I can't vouch for this working without any problem. If you're the least bit nervous or suspicious about this remedy, call Tech Support and I am sure they will walk you through this. I was on hold for about 1 minute, and although it was tedious to fix the problem, all was resolved in the end.
Note: This procedure will reset many of your preferences so you will have to spend some time resetting Finder preferences, etc. Kind of annoying but at least everything works