I have done a lot of showers (3) and I've found some really exciting things to share with you.
First my main concern was the cost of the drywall behind the Kerdi. I mean you can spend $30 - $50 dollars for that drywall. Well I've solved that problem!
The other day I was at the liquor store. Ya know, they have a lotta boxes... Well I got a boat load of them, and took'em apart real careful (it's more relaxing to listen to some Deliverance music). So I stapled the boxes to the studs - did two layers, and made sure not to overlap the seams.
Then I had some left over varnish that my son hadn't huffed - and applied it to the cardboard. I figger a nice waterproof substrate. So right here you've saved over fiffty dollars!!! Then that unmodified thin set ( hey it's 1/2 price cause it was 5 yrs old) - stuck that Kerdi real gud.
I mean since the Kerdi stuff is waterproof - ya can go right over dat cardboard!
And hey, it worked! We've been using the shower for a couple a days, and everything is fine. We may even put up some tile.
John, don't feel too bad gettin' adivice from a fella like me. I'm sure in time you'll be expert like me.