That is to ignore joystick inputs near the center so the car stops when the stick is released, even if it doesn't always spring back to the exact center reading.
This controls how much faster one wheel goes than the other. Use lower numbers if the car turns too quickly. Use higher numbers if the car doesn't turn quickly enough.
What analogRead() values are the X and Y producing?
It should be. I saw in the user manual for your EZRUN MAX 8 BLDC motor controllers that your controllers won't switch to reverse until the car has come to a stop. You may have to add code for that... Keep track of which direction your motor is going and stop the car if either motor needs to switch from forward to reverse.