While you can have river rock as the floor, industry standards won't allow you to do what you are proposing. They don't consider concrete as a waterproof layer, so you need a liner. That liner must be sloped to a drain with weep holes to drain any moisture that gets beneath the setting layer. You can still have a barrier free shower, but you shouldn't (and it wouldn't pass code if the inspector knows anything).
So, I'd have them depress the slab where you want the shower to accommodate the barrier free, then build it up 'normally'. This means, preslope, liner, setting bed, then finished surface. Getting the details all right with the walls as you've envisioned might be a little non-standard as well, but doable. Suggest you check out www.johnbridge.com for help on this...they deal exclusively on tiling, showers, etc. and lots of tiling/shower pros to bounce ideas off of.