As to the backer, because you are going over drywall, nailing like crazy won't do any good, because only a few will hit wood behind the drywall. I'd suggest going with 1/2" HardiBacker instead of Durock because of this; I think it has more "stand-alone" internal strength than the Durock. Find your stud locations prior to installation, use lots of construction adhesive, and screw the backer to the studding every 4 to 6 inches(through the drywall). Be sure to check the construction adhesive you get will adhere well to cementious products.
So long as your stud centering is every 16" or less, a good construction adhesive bond between the drywall & backer will take care of the interim areas where nailing won't work.
What type of cultured stone are you looking for info on?