Water pump that will not block water in case of power loss
A simple solution would be to install a full sized bypass with a check valve, which ties in downstream of the pressure tank. This would allow full system flow in the case of pump failure. You could then use a pump of whatever type fits your needs most efficiently and effectively, and would need not be concerned about flow restriction through the pump. 

You do not mention the nozzle activation mechanism but we must assume that the heads are closed unless opened due to fire melting a fusible/frangible element at the head(s) or other mechanical means. So... under static conditions the system would remain at the pressure held by the tank diaphragm, which would be above mains pressure (because it was pumped). Upon opening/activation of a spray nozzle then the tank would deliver until pressure drops enough for mains pressure to then lift the check valve and flow into the system, around the pump and tank assembly.



sorry for crude drawing of bypass with check valve