yes, you do want to be able to power off the board, put the switch in the black wire and run it to AC1 , Operationally there isn't any difference between ac1 and ac2 The ground does go to the case. Also bring a ground back to the case from the motor as well
Placing the switch on the center lead of the Pot leaves potential available to the speed diodes there is a possibility for damage to the board if connecting motor wires etc without shutting off the power. There is also a resistor that can be cut that removes the need to reset the speed to zero each time you power the off the board, that is what I did with mine,
I also leave my power/speed set and switch my forward to reverse back and forth when I'm power tapping on the mill or lathe and allow it to stall the motor then reverse to clean the tap. I vary the max power or speed with the size of the tap. The acceleration curve allows this to work without breaking the tap.
Art B