OK, I'll bite
SO the boiler feeds the pump which feeds the manifild that has control valves. The zone controller looks at the thermostats and decides which zones get heat, up to a number your pump can support? Then the controller fires the boiler which activates the pump? I am not quite seeing the purpose of the "end switches" Do you have a drawing?
What if all the zones need heat at the same time? How does a pump that is not sized to feed all of them at once deal with this? How is the boiler sized to deal with the switching zones? IE: Your heat loss says you need X ammount of BTU/HR to maintain a certain temp. So the boiler obviously needs to be this large, but if the heat is being pushed to different areas over time, don't you need more heat capacity to deal with this switching(push more heat into the zones that are on)? Concrete takes a while to warm up, while you are heating one zone another is cooling off. It seems to me that this would lead to unsteady/overshooting heat conditions, kind of defeating the stability benefits of warmed concrete.