Accidentally connected 12V into the USB power input. Saw a puff of smoke. The 3V3 is now shorted to Gnd. I’ve removed the regulator. Does this means it is toast? Is there a way to try to recover or check that it is still good?
Probably damaged.
Someone correct me if I’m mistaken, but I believe you could try supplying it with 3.3V on the respective pins, and see if that works. If you’re lucky, you’ve only blown the regulator.
It depends if the short between 3V3 and GND was measured before or after removing the regulator 
If @macbum got the short after, I think It’s time to say goodbye 
The specs for the reg - 2.5V to 5.5V Input Range. Hopefully it didn’t blow through
Confirmed toasted! The Vin of the regulator is shorted through to the Vout thus killing whatever is connected to the 3V3. Dammit!