In this case, it's easy - he has two widebands and one is saying lean, one rich. In the case of the PCM and fuel trims, it uses data from the two front 02 sensors. The rear 02 sensors are not in the equation, they are there to monitor catalytic converter performance. To put it in really basic terms, the front 02 sensors switch around the stoich point of the fuel used and the PCM makes corrections based on the variance from stoich - at idle and part throttle in closed loop when it's at operating temp. At WOT it uses data mainly from the MAF and base fuel table to apply a global fueling to the engine. From the factory it can apply certain long term corrections to OL fueling, so in essence it does use 02 sensor data, but not directly - and in most cases, this is not a good thing and is usually changed when a car is tuned.
As far as one injector is concerned - it does not, it takes data from the gas coming down the pipe to the 02 sensor. If one injector is bad, it may or may not be enough to get the system so far out of whack it throws a CE light. If it is just happening at WOT, they it doe not know at all. A good check is to datalog 02 sensor volts and see what is happening - or use a Snap On Scanner to look at individual injectors performance.
That all applies until 2011. 2011 and up Mustangs run in closed loop all the time because they have factory wideband sensors in each bank that keep the system stable.
Bottom line - first thing I'd do is replace the lean wideband 02 sensor.... Then start looking at the motor.