Tara on "Latest version 500 error distaster"

try MANUAL updating. Download a fresh copy of the WordPress .zip file to your computer, unzip it, and use that to copy up all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory. You may need to delete the old copies of files & folder on your server before uploading the new ones. Read the Manual Update directions first.
- If you haven't already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong.