You might try a rotary hammer to drill holes in the rock. It might be easier to break it up with holes in it.
With the irrigation the easiest way to tell if you have the needed water flow is to remove one spray head from the zone and temporarily connect 8 heads in it's place. Won't be perfect, but it will give you an idea how well it will work with 11 heads. There are reduced flow heads available if needed.
It sounds like your installer ran pipe from the valves to each zone. I don't know of much you can do about adding a zone beyond the valve. If I understood wrong and the valve is near the heads you could convert to a 2 wire system, but it would be cheaper to run a new pipe.