#1 Left is negative, right is positive. There are 2 ways to reverse it. One is in the ports and pins, motor outputs, dir active low. Change it. You can set X=0 anywhere on the table you want.
#2 Motor tuning, steps per unit. 6 x 200 x 8 = 9600 steps per unit. You'll have to retune the motors once you change this setting.
#3 Fixing#2 should fix it.
#4 Your limited to whatever you can fit into 1000 lines of g-code.