Personally, I'd not use a conventional liner and use a surface applied sheet membrane...something like Kerdi or Hydroban sheet materials. On the cement walls, you could apply the membrane to it directly and not lose any space in the area by building it out. It's an easier install for the pan as well, since you don't need a preslope, then install the liner, then another mudbed...you just need that first sloped pan, then install the membrane on it directly. Then, you end up with an entirely waterproof enclosure, rather than a water resistant one with a pan liner that can trap moisture in the 1-1.5" thick mudbed above it (especially if you don't keep the weep holes clear!). If you don't use the preslope, you're asking for problems that can show up in as little as a few months (i.e., don't put the liner flat on the floor then build your slope.
Since you won't have studs you can notch, yes, your folds can be an issue...this should not be an issue if you use a sheet membrane to waterproof the entire thing. Plus, because there isn't a cement layer underneath, the whole shower will dry out quicker. FWIW, neither tile nor grout are waterproof...some moisture will get through. On a wall, it tends to evaporate between uses since gravity helps most of it to sheet off (but not all)...on a mudbed pan, even with proper weep holes, it stays somewhat damp after the first few months.
Using a sheet membrane isn't all that difficult if you follow the instructions...it's sort of like putting up wallpaper, but using mortar (thinset) rather than glue.
Check out
www.johnbridge.com where they specialize in tiling things, especially showers.
