So the wire to the 12 volt battery was not connect tight. How many times have we had to wack a battery terminal on a ICE car to get it started.
Did they say what the voltage was on the 12 volt battery. Battery Dead and voltage too low to allow the car to start might not be the same thing. From a quick read of the posted service manual the car seems to monitor the 12 volt current and a LOOSE connection might be what triggered the Battery light and some of the codes.
As they said gained access they must have been at the fuse in the back of the car by the battery terminal.
This might be one case where trying to jump start the car from the back would have found the problem sooner.
2012 RED Volt VIN:#C-8860 - Premium Leather Seats Jet Black with Dark Trim, Rear Camera & Park Assist, Nav
09/29/2011 Ordered -- 11/15/2011 took it home