all good. You are getting there.
jd,
Everything Jim deB said is correct. I will add one more piece of information about what he said: that 1/4" Hardiboard is OK for walls too, and the James Hardi company has always said so. It is on their web site. They used to sell only 1/4" Hardibacker board. The newer "500" series costs a lot more, and isn't 1/2" thick so it serves no purpose in terms of getting a wall flush with the adjacent 1/2" gypsum board. People used to use strips of 1/4" Hardi as furring under 1/4" hardi borad to build out the wall to meet the adjacent wall. Some still do.
Hope this info helps, and does not cause any confusion. To summarize, the conclusion to draw is that 1/4" Hardi is sufficient everywhere, and no thicker Hardi is required for any reason. Some people do 'spec' it, and that is their choice.
Now, on to the next topic. Kerdi is your only option in a shower or wet area. Ditra is not an option, and nothing that Schluter says will contradict this. A Kerdi drain is big and needs space below it. If you order your Kerdi products from the "highly recommended" supplier you will get real high level advice about anything you need, all the time. He uses modern courrier delivery services, and is always available on the phone. So I do recommend you use him.
Before anyone recommends that you have plywood installed sunken down low between the joists, can you tell us first what the facts are about your floor levels?
I don't recall reading whether you have any plywood installed or planned to be installed ABOVE the diagonal 1"'x8"s that you mentioned above. This worries me.
Your initial description of what your contractor was intending to do also worries me a bit, as it seemed that he might be planning to "mud" (i.e. thickness = 1" or so) ABOVE the Hardi CBU. That is a no-no.
Hope this helps.
David