Re: What thickness cement board
Schluter guarantees kerdi to be waterproof and they encourage its use directly over drywall, although many people use Durock anyway (belt and suspenders). I personally hate hardibacker, as it absorbs too much moisture; it draws the water out of the thinset too fast.
The Schluter foam shower pan/kerdi system is awesome. And don't forget the Ditra on floors. It is insane to use a CBU under Ditra on the floor, unless you need to make up the height.
Remove not the ancient landmark, which your fathers have set.