Flooring Tile - behind tile is shower area

I will try not to make this confusing.

We are remodeling a bathroom and built a custom shower. The bathroom is cathedral ceilings now but the shower area is boxed off. We are using the Schluter Shower System. We put all the Kerdi paper on walls floor and ceiling of the shower area. 4 days later began to tile the ceiling and the tiles were not setting flat. Pushing up on the ceiling we found alot of play. (Sheetrock under kerdi) trying to prevent later problems we decided that we should take ceiling down and put more supports under the sheetrock. When we did we found that the sheetrock was saturated, in fact some area acutally puddled. Not being happy, but trying to prevent future problems we cut a hole in a side wall (outside wall) and found that to be saturated also. Assuming condensation, being that the insulation is dry. They had applied plastic as a moisture barrier. So the layer went as so. (inside out) Kerdi paper, thinset, sheetrock, plastic and insulation. Visiting building supply and tile places I hear conflicting issues, not enought ventilation, shouldn't have the plastic, you need the plastic, shouldn't use sheetrock and kerdi, should be using durarock (cement board).? Now as I sit I have 1 outside wall and the ceiling down, 1 more outside wall is still up. Now with the short story long, what do you think?

Should I just use durock??


Thanks

No plastic on exterior walls behind a waterproofed system, it will trap moisture.:)

Cut slits in the insulation if faced to let it breath.:)