I installed a delta univ valve in my shower last year. (w/ monitor 1700 series trim)
I believe you should leave the plasterguard on and mount the valve such that the face of the plasterguard is flush with the finished tile face - not the cement board. So, hold up yr backer, and a tile and any other membranes you're gonna use, and then secure your valve.
So, you have to cut a 4x4" hole in the board to accomodate the valve AND the guard. If you butt the guard on TOP of the cement board, it'll bring the valve too far out such that your handle and escutcheon will ride above the finished tile surface.
The escutcheon on the delta trim will cover the square cutout sufficiently.
(Remove the guard after tiling but before you mount the handle and trim. )
I found it helpful to plan access from the other side of the plumbing wall. That made mounting and jiggering the valve depth easy once the backer and tile were up.