neksmerj:
I have no idea how this could be done.
This is the problem. I can try to help you sort out how to do things but you have to have something that forms the basis for a question.
What you have described seems eminently feasible. Are you confident that you can do the mechanical construction?
One of the links I gave you earlier has code to make a stepper move back and forth in response to push buttons.
Most stepper drivers have ENABLE pins which you can activate to de-power the motor to allow it to freewheel.
Stepper motors are inefficient. I hope you are planning for a big 12v battery.
...R