Walk away and never look back, the dude above me is completely right, I agree with him 100%...
Dealers have access to spark plugs all the time, such B.S. that they say they don't have time... they be trying to burn you man!
I know it must but tempting, but you may regret it!
Anyways, if you think I am full of B.S. and just jealous of your find...
Go ahead and do the test, you may get very lucky and have your self a nice new car!
As for the spark plug question.. .it makes sense to me that you could leave all the spark plug slots empty since they are indepenent of each other... when testing for compression.