I would think that you would want to switch the pump between "low speed" and "high speed" with a simple fixed value electric thermostat. Wire the speed control to give high speed as the default if a wire breaks. Put a switch on the panel to select "Auto" (use the thermostat) or "High" (bypass the controller and thermostat and run the pump directly on 12 volts.)

Might be nice to have an indicator on the panel showing the actual pump status (High, Low, or Off)