Simply safemoding, restarting, and reloading the update turned out not to be enough, but I do thank you for pointing me in the right direction. I ultimately found what I think was some help at:
After doing all three of Topher Kessler's suggestions, my Mail was finally running again. It was still a mess to clean up and restore all the various mail folders that were found in my Library, but the brand new Mail preferences file and cleaned up password keyring seem to have at least let me access my ISP for outgoing mail.
Thanks again for pointing me in the right direction.