Never use sheetrock in shower walls - use 1/2" cement board instead, with taped seams. 2 cm is pretty thick (about 3/4"), so I would check with the manufacturer about the appropriate substrate thickness.

Hardiebacker cement board is fine, but you still need to check with granite manufacturer for recommended thickness.

Those are thick slabs for walls I too would check with granite maker as to what you need to make sure this works

Walls - What about the floor, would that have to be re-inforced to hold weight not normal in a shower.