Replacing upstairs Ensuite shower (Kerdi)
 

My house is 20 years old. I tore out the old tile shower when I noticed one wall was soft. That was an outside wall and underneath the sheetrock was the plastic vapor barrier. It was covered in black mold. I've removed everything right down to the studs and plywood floor but I didn't remove the plastic vapor barrier from the outside wall. I've reinstalled concrete board on the outside wall (opposite the showerhead) and greenboard on the rest of the shower enclosure and I've taped and mudded all the seams. I purchased a Kerdi kit with the membrane and tray. Before I go any further, I have a couple of questions:

• Can I apply thinset and Kerdi overtop of drywall compound?

• As I've heard that should be some way for the water from the thinset to evaporate, can I apply Kerdi to the wall with the plastic vapor barrier still intact behind the concrete board?
  If not, do I have to rip out the board and remove the plastic or could I just apply the Kerdi partway up. That wall is 92 inches high, so could I apply Kerdi to just the bottom 60 inches?

most tile installers don't like to install anything over drywall compound.
but you can prime it and go over it with thinset.

you will be using a unmodified thinset behind the Kerdi so you want the thinset to slow cure with moisture to make stronger.

I don't think the plastic barrier needs to come down since when Kerdi is done correctly there will be no moisture getting behind the wall.

others here may have more insight on the barrier for you.