Three suggestions: one, forget greenboard (use the durrock or see suggestion later); forget a copper pan; forget building up the floor with wood.
Check out www.johnbridge.com for showers and tiling help. They have a bunch of people that have built barrier free showers and you'll have a bunch of tiling pros to help out.
The waterproof pan MUST be sloped to the drain...while possible with copper, it is very difficult and there are other materials better suited. You don't really want any wood in the shower,either. Copper pans seem to be an east coast (northeast) thing, and are rarely built to code (which requires the waterproofing to be sloped - most copper pans are installed flat on the floor, not what you want at all).
Seriously consider using Kerdi from www.schluter.com to waterproof the shower. This would allow you to do the walls in regular drywall (eaiser, cheaper, faster), since the membrane creates an absolute waterproof layer, no moisture can get to it. This also means that the shower dries out much faster since you don't have stuff behind that can absorb some. To help, the entire floor outside of the shower could be waterproofed with Ditra from the same people. Normally, a handicapped access shower has a slight hump outside that directs any water into the shower, but can still be rolled over easily.
To allow multiple shower heads, you could either use multiple valves, or a multi-port divertor to switch between them. Get the right valve, and you can have any combination of heads activated. I'd also consider a thermostatically controlled valve, since you could set the temp once, and then just have to deal with the volume/divert issues, and, the temp would be stable winter to summer as the incoing cold water temperature varied (often quite a bit).