VICTORY IS MINE!!!!!!!!!
Ok seriously, this should be stickied for mac users. It is ABSOLUTELY IMPERATIVE to download the android sdk from google - Android SDK | Android Developers
DO NOT use the packages you see floating around in various fastboot guides for mac where people are using fastboot-mac files. Download the latest android sdk with 4.3 updates.
After I did that, everything worked like a charm. The length of time it sat on "signature checking" made me go crazy. Just like your write up said, I got the pre-update error. Flashed again and it finished - total time 361 seconds.
Thanks guys, phew - I've had a crazy amount of devices running different roms and this made me really nervous (because I was using mac and getting those connection errors when trying to flash something like a ruu - scary).