The way you describe the two sooty black plugs, came across to me as if those cylinders were burning rich. Maybe
oil? It's hard to say from a description.
Since it is EFI, if the outside cylinders were sucking air in at the intake ports, that air would not be measured by the MAF sensor, and the outside cylinders would burn lean. If in the close loop mode, the O2 sensors would see lean and it would richen up all four cylinders on that bank. This would cause the inside two cylinders to burn rich, if they were not leaking in extra air, as are the outside two in this scenario.
Is this a MassFlow as in it uses a single plain carb intake with injector ports welded in and a throttle body that looks like a carb, where there is a GM MAF meter inside the air cleaner? If so and the intake gaskets are leaking, it is a real possibility that the MAF meter is coated up with
oil.
Dumping way too much fuel can wash all the
oil of the cylinder walls and cause ring wear. The compression seems to be the same on these cylinders so that does not make sense.