I decided pin 12 looked like a good candidate, and hooked it to a button wired to 5v.
You have removed the button I trust.
5v on pin 12 shouldn't have harmed the card. There is a buffer between the pin and the card to shift the voltage to 3.3v.
On the other hand, if the Arduino is trying to use that as an output pin and you short circuited it with the switch, it could have overloaded it and left you with a 'dead' pin. To see if that is the case, write a simple sketch to blink an led on pin 12 and connect an led to it. If it doesn't work, you can probably repair it with a new Atmega328 chip. https://www.adafruit.com/products/123