This is a really awesome project that you have taken on, but I can't help but wonder why you bothered with sheetrock if you covered it with plywood. And I do hope that the sheetrock is the kind that goes in a shower. Otherwise you may end up with a mold problem.
All in all, it's a great looking project and I am looking forward to finished pictures.
Honestly, I wondered the same thing. My husband insisted that it would give the greenrock extra protection, therefore giving the studs under the house and in the wall and extra layer of protection. The scrape/discolored spot in the second picture was were water had been getting thru and the water had destroyed the sheetrock. We wanted to make sure that kids would not be able to push on the wall to hard and break the shower. The only downside is that if a pipe burst I will have to go in thru my bedroom wall to fix it