A vapor barrier behind the cbu lapping over the tiling flange is all that is required. Some people prefer a surface membrane to waterproof the wall instead of just making it water resistant. There are lots of choices here, I prefer Kerdi from
www.schluter.com, but Noble makes some good stuff, along with Latticrete, and there are paint on waterproofing membranes like RedGard from Custom Building Products that work as well. Grout and tile are not waterproof, very water resistant, but some will get through. Then, it gets into the cbu. If it doesn't get a chance to dry out in between showers, it gets to the vapor barrier, which when done correctly, gets back into the tub. Blocking it at the surface under the tile is an other option. Don't do both.
A glass panel at the end sitting on a knee wall would be thinner than a true stud wall, at least on the upper area. Depending on what you were going to do, that is a possibility. Another choice would be a glass block - the thinner ones are 3" thick, I think, which would be thinner than a wall with sheathing and tile. If, when you build the wall, you make the shelves in the storage area rigid rather than moveable (or at least a couple of them), you'd go a long way to stabilize that wall. Otherwise, I'd still prefer to see a thicker one to help stabilize the tile.
HardieBacker states that you can use 1/4" wallboard on the walls, but most pros much prefer the 1/2" on a wall for the extra margin. I don't think any of the other brands suggest anything thinner than 1/2". 1/4" is all you ever need on a floor, although you can go thicker to help match heights.