I would have to concur that you should have a drain drop at the end of the line on either side, but that should be all you need in a two-car garage, IMO. If you were plumbing a much larger shop that had a high-volume usage and multiple user points on each wall, as all the diagrams you posted show, then you would need more.
As far as putting in all those expensive unions, I don't believe those to be necessary. You should sketch out your actual needs and then only put a union where you feel you might have to make a major disconnection in the future. Even most production shops that are plumbed for air do not have all those unions due to the cost and relative non-necessity.
And yes, you should be able to reduce the pipe diameter to 1/2" for the actual drops. The 3/4" main lines are to give you the continuous volume you need to run more than one line at a time. This is assuming that your compressor has the air volume capacity to run two lines at once.
Above all, don't go all Samuri on it and K.I.*.*.
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s