What's left of your shower pan would not pass industry standards. The liner first needs to be sloped to the drain (can't tell for sure, but it doesn't look like it), and, the liner needs to come up and over the curb, which you've demolished. The whole pan seems elevated more than I'd like, but that may be because they needed the height for the drain. To make the whole thing lower, there are newer methods that would not require a second layer of mud over the liner...the liner becomes both the waterproofing and the tiling surface. There are 3-4 companies that make that sort of system, and it is a system, best contained within one company's products. One of the older ones is from
www.schluter.com, but Noble, Laticrete and others make a system that will work.
The mudded wall is still a preferred method to ensure that the wall is (or should be!) perfectly flat. Otherwise, that depends a lot on the quality and installation of the studs as things like cement board (cbu) will bend to conform to warps or twists of the structure, and, the corners may not be square, either.
Personally, I'd want to tear out the pan, inspect the plumbing, and rebuild the whole thing.
FWIW, the pan (should) be made out of what is called deck mud...a mix of 4-5:1 of sand
ortland cement and enough water to make it stick together. It doesn't flow...think wet beach sand like material that you can shape.
You can check out
www.johnbridge.com where their whole site is devoted to tiling things, which showers constitute a large proportion. Lots of pros participate, and probably faster answers, too. This site is great for some things, but for tiling things, I think that one is better.
