While many of your comparisons are inaccurate, alot of this does go on. Most of the GM models share a large quantity of componants, and usually the bodies are only slightly different. It's all part of making the manufacturing process less expensive. Just like huge metal presses can be fitted with different dies quickly to modify the output, many of the manufacturing lines themselves have the equivalent of a "Chevy / GMC" button on them.
One of the most impressive examples of this was a line that took the inner "sound dampening" portion of the hood, and attached it to the outer "looks nice" portion of the hood. Several different models could be produced by slightly changing the input material, and flipping a switch / pressing a button. The extra mile that goes into manufacturing the higher end domestic vehicles (Cadillac, in my experience) as well as the decreased tolerances for the parts really do make that extra dollar worthwhile. I remember when some cement was being removed from a shipping dock with several thousand 'Vette body parts nearby. A little dust got on them, and they had to be thrown away, no clean, no inspect, just melt 'em down and start over.