The first thing that strikes me is that motor controllers usually need 3 inputs to control each motor.
The first sketch from the OP defines 4 pins (8, 9, 10 and 11) as outputs
pinMode(8, OUTPUT);
pinMode(9, OUTPUT);
pinMode(10,OUTPUT);
pinMode(11,OUTPUT);
In the first sketch posted the 4 outputs are used as follows
digitalWrite(8,LOW); // motor A back
digitalWrite(9, HIGH); // motor A back
digitalWrite(10, LOW); // motor B back
digitalWrite(11, HIGH); // motor B back
In the next sketch 4 pins (8, 9, 12, 13) have been defined as outputs
pinMode(12, OUTPUT);
pinMode(9, OUTPUT);
pinMode(13,OUTPUT);
pinMode(8,OUTPUT);
and used in the sketch
digitalWrite(12, HIGH); //Establishes forward direction of Channel A
digitalWrite(9, LOW); //Disengage the Brake for Channel A
digitalWrite(3, 255); //Spins the motor on Channel A at full speed
digitalWrite(13, HIGH); //Establishes forward direction of Channel B
digitalWrite(8, LOW); //Disengage the Brake for Channel B
digitalWrite(11, 255); //Spins the motor on Channel B at full speed
but pins 3 and 11 are not defined as outputs so will default to being inputs so will not behave as outputs
At no time are 6 pins (3 per motor) defined as outputs and used in the sketch.
One useful thing to do would be to give the pins meaningful names which would make reading the code easier.
I am in agreement with John that the full sketch be put aside for now and that a start should be made with small steps.