Hollis,
The component preventing your hydraulic pump from extending the gear is the pressure switch.  It is either encountering enough pressure to open or is stuck open.  Given that you have replaced the switch, let's assume the switch is not sticking.  The case you describe of trying to lower the gear while it is retracting is the easiest test to see if your spool valve is installed backwards.  Your results of this test indicate that it is indeed backwards.  To be  100% sure without taking the pump apart, you can put a pressure gauge in the down circuit.  During retraction you should see virtually no pressure.  Another, even easier but not so scientific, method is to measure the resistance across the down pressure switch to determine if is remaining open during the retraction cycle.  If the spool valve is in backwards you will read the relief valve pressure setting for the down circuit during the retraction cycle.  
The purpose of the spool valve is twofold.  First it pushes the poppet valve, which is retaining system pressure, off it's seat allowing fluid to return to the pump.  Secondly, for the retraction stroke, it allows the rod volume to return to the reservoir without restriction.  If this path is not available, the rod volume must push past the pressure relief valve in the down circuit.  Your high pressure circuit is then actually pressurizing the low pressure circuit and opening the down side pressure switch.
There is another very rare scenario where the pump turns on for a fraction of a second and then stops, but based on your description of the ground test in particular the spool valve seems like a likely culprit.  A large number of pumps are known to have been built up incorrectly.
Chris Zavatson
N91CZ
360std


Christopher Zavatson
Mobility Technologies
United Defense
(408)289-4329