Mtruly,  To test the power supplies and motherboards AND cables you would need to have a known good setup, then use the working power supply to test the slots, while testing the power supplies on a the working slot.  So to answer scenario 2, yes you can.  Yes you are correct but I would still test both slots with a known working power supply to confirm. If confirmed bad then you can run it like that and if there is a slot 2 failure the board would have to be replaced. I would also make sure it isn't a bad cable as well. 
I have some older Poweredge 1950s that run with a single power supply unit installed. After many years of operation, the servers may not power up.  I look at the LEDs on the power supply unit and they are off.  There are two possible scenarios. 1.  I take the problem power supply unit out of the server and plug it into a good working slot of a different server and if it doesn't work... it tells me the power supply unit has failed. I have to buy a new power supply unit. 2.  I take the problem power supply unit out of the slot of the server and put it into the other (empty) slot and the server works again.  I can continue to run the system. My question is... in this second scenario, am I correct in assuming that there is a failure of the motherboard on the first power supply slot and in the future, if the second power supply slot fails in the same way... I will have to buy a new motherboard to get the server working again? Thanks for any ideas!
Poweredge 1950 Power Supply Question