Does a showerpan curb need to be poured sand mix or can you just frame with 3or4 2x4s and wrap with membrane and durrarock? I am going to put a bench in the stall. Does the membrane have to go up the bench face then on the seat and up the stall 6 to 8 inches?

You do not use Durorock on the floor of a shower. Take a quick read of this fellows approach to building a shower pan. How to Build a Shower Pan