Adding "catastrophic upgrade" to my lexicon

Fixed it! I have a bone to pick with whoever decided that the interface for a failed OS upgrade should include nothing but a "restart" button which leads back into the exact same failed OS upgrade.

The magic sequence of operations to recover from that, discovered after many iterations, is

wait for the 45 minute bad installer to completely finish failing to install -- interrupting it will prevent the next step from working;
zap the PRAM twice (not once, not three times) by holding down opt-cmd-p-r during reboot;
hammer on the option key as quickly as possible throughout the third reboot to get to choose a different installer. Which had to be on a USB drive.

Oh and you can't use a wireless keyboard for any of that, and have to be hardwired to the network for some reason, wifi isn't sufficient, and about half the time the computer won't talk to the monitor after a reboot so you can't see what you're doing and have to start the whole process over.

Computers, they make our lives so much easier