Welcome, Martin.
It's important to know whether your original subfloor was 1/2" or 5/8ths" plywood. The 5/8ths may have proper T&G edges, while the 1/2" does not, nor is the 1/2" suitable even to be a first layer of structural subflooring. The orientation of the second layer is also important.
You are using a
tiile-redi shower receptor, rather than a "ready-tile" product?
1. Depends upon the application. First, your floor does not need to be level for a ceramic tile installation, but it needs to be very flat. The use of an unbonded, reinforced mortar bed (minimum 1 1/4" thick with welded wire mesh in the vertical center) is a very good way to achieve both if you have the vertical space and floor structure to accommodate it. There is also an industry accepted method using a minimum of 3/4" of mortar over a cleavage membrane and expended metal mesh that might work in your application. Or, you could use a SLC if your application permits. Most of those also require a reinforcing mesh of some sort and have a minimum thickness requirement. You could also consider just using a Cementitious Backing Unit (CBU) or other tililing substrate.
2. Be careful whose YouTube videos you watch. The manufacturer of your shower receptor (If I guessed correctly) requires a cleavage membrane of some sort over the plywood subfloor and under the mortar bed for his receptor installation. I recommend you read and follow the manufacturer's instructions for his product if you want to use it. Creating your own shower receptor can be much, much less expensive is easier to fit your exact shower footprint and drain location. You would still want a cleavage membrane under the mortar used for the pre-slope or single mortar bed in that case, but the requirement for the floor to be dead level doesn't apply.
3. First, I recommend you not use a marble on your shower floor. Type marble shower into our Advanced Search feature and read a little for information on that.
As for the floor structure, we'd need to know a good deal more to about it to judge. You can enter your information into the Deflectometer in the dark blue bar near the top of the page for an initial go/no-go reading on that.
4. The bathroom floor is not usually considered a wet area, but if you want to consider yours a wet area, yes, you'd need to consider some sort of waterproofing. It's more complicated than just painting a liquid membrane on the floor, though, and may be more than you actually need.
As for the use of other types of membranes, such as those designed for radiant heating systems, you'll again want to read the product manufacturer's instructions.
My opinion; worth price charged.
