I have no interest in going over to the JBF site; I might go back later when I feel up to it.
- Question 1.) We have almost enough information to be able to answer it.
- Question 2.) Easy. Use a glue or cement, depending on what the maker of the underlayment suggests.
- What is Question 3. about? Thinset mortar for tiling? A possible mortar to bring the floor areas to the same level?
About adding a CBU on top of concrete: NO. There is risk, and there is no advantage and no gain (except the "gain in height"). If you want to thicken your floor, gaining that much height, with a hard cement product, just use a dry bagged cement product that you mix with water and spread out with a trowel. It costs less than CBU too. Call a cement manufacturer and they'll tell you about patching mix, resurfacing mix and grout mix. (So, AFAIK, CBU is for wood substrates.) Call Hardie or view the web site to see if they warrant tiling on CBU on concrete.
You have two distinct floor areas and a door between them -- is this right? Are you planning a door sill? If you could put that in the plans, it will give you a few big advantages. For one, you now can do what is right for each floor area, and let the sill plate (e.g. marble) handle the break. Each floor area gets a tile layout that is right for it. Your one area gets the underpadding or filler treatment that it needs. If you ignore all this, I believe you can still tile over the break in the two types of underlayment and not risk a crack, but that depends...
In 2005-2006 I spent time at that other site. No-one ever gave a straight answer about tiling on a concrete surface. More than one key player talks about adding "mud" on top of concrete slabs if no D*tra. In other words, no-one says "no" to alternatives but no-one confirms. It appears as Either you add "2 inches of mud" or you put D*tra down.
Truth is, you can tile directly on top of a concrete slab or on top of any underlayment made for tile that you peel-and-stick, glue, or thinset, onto the concrete. At that site, all the non-orange membranes (or underlayments) get short shrift since they give more options to the newbie DIY who may then not buy the big orange membrane in the sky. That's in their DIY forum. In the Pro forum inside JBF there is a lot more truthful discussion of the issues, but Pro's are gently warned not to "confuse" the general public DIY. This keeps the Pro's under control, segregated and partially muzzled (in return for freebies). But even in the Pro forum, there is a lot of voluntary self-censorship. Cork has been used as a tiled floor membrane for centuries, and they make it seem risky, never-heard-of-in-these-parts, likely to fail and likely to rot. Other synthetic products get ignored; some get "confused". Orange membranes are worth a lot of money to them.
In your application, if you want to buy a synthetic membrane, I could name a dozen that will all work. You're on concrete after all, and that is the best starting point for a tile floor!