Diagram and description are correct. Pump --> 18mm on HB. 16mm on HB --> pressure port on Rack/box. Return from rack/box --> cooler (if used) or pump.
I believe only reason for different size fittings is to prevent careless mechanic from crossing lines.
Original HB systems have a separate return to the pump body for the HB return line.
I have used a Tee but it seemed like pressing the brake during a turn would cause steering assist to drop momentarily. Either way I would recommend a cooler before the return gets to the pump. HB can create a bunch of heat.