I looked at my 220d the other day and sure enough, the metal pipe goes to the center of the water pump, not the edge as I thought all these years. So, yes, the metal pipe is the return line and my initial posts were wrong.
The metal pipe is indeed the return line.
The coolant is supplied from the lower radiator outlet, enters the system through the lower hose housing, flows into the water pump cavity where the impeller (it's not actually a pump as it doesn't compress anything) creates the flow through the coolant passages and exits via the top radiator hose. The thermostat, located in the housing connected to the upper radiator hose, controls the flow by opening or closing.
The heated water is supplied by an outlet from the intake manifold. This is the valve located on the firewall operated by cabling connected to the heater valve controls in the dash (the "sliders"). The water enters the valve through the center formed hose. The position of the valves determines if and how much heated water passes through the heater cores (there are two within the actual heater core unit, driver's and passenger's sides) and exits through the return pipe. Minimal flow, if any, occurs through the return line when the heat valves are in the "off" condition.
The coolant system pressure is created by the expansion of the heated coolant and not the "pump".
Heater core. The line on the LHS of the picture is the return.