The return line will always be the uppermost connection on any type vehicle where fluid is circulated outside the mechanism for cooling.
Unless I misunderstand this, I'm not sure the return line is always uppermost. Think of the cooling system. The radiator, through which coolant is circulated, is outside the engine ("the mechanism"); nevertheless, the return line (coolant going back to the engine) is at the bottom, as it is on virtually all cars.
If we look at the transmission cooling lines in the diagram in post #7 above, we see that the outlet from the transmission to the cooler is just above the filter, and the return to the transmission is at the rear at almost the same level.
The flow through the ATF cooler lines is push, not pull. There's nothing on the return side to pull fluid back to the transmission. This is easily verified. Disconnect the cooler lines. Direct the outgoing line (to the cooler) to an empty container. Connect a hose to the return line and place the end of the hose in another container full of fresh ATF. Start the engine. The empty container will fill up as fluid is pumped out of the transmission, but no fluid will be drawn back in from the full container on the return side. This is a method often attempted to replace all the ATF, but it doesn't work because the transmission will pump out but not back in.