I do not think you should have to mix the channels in your remote. I do not on a rover with skid steering, which is basically what you have.
You should set the SERVO1_FUNCTION to 73 and SERVO3_FUNCTION to 74. (which I think you have done)
Then Ardurover will handle everything.
Be aware, though, that you have to set the RC1_REVERSED and RC3_REVERSED to 0 or 1 to make your motors spin in the correct direction for forward and backward motion.
And, you configure the type of steering method you as the operator wish to use with the PILOT_STEER_TYPE. You might look here: