You ask some very interesting and difficult questions. The Mercedes Benz Club of America judging standard is, as you type, just the way it rolled off the asembly line. Which may not be up to top concours standards. The 250 SL that was recently restored by Hjeltness Restoration (and was on the cover of the UK MB Classic mag) would be a case in point. They were meticulous in bringing the car to factory level. One item is they recreated the paint runs on the inside of the hood. I try to stay with the standard of the way the factory did it, not with extra time.
The underside of the car would be finished in primer/body schutz, and mostly would not be fully car body color. There would be overspray along the edges, but sort of a greyish/brown for the rest. The big plate under the transmission would be full body color. When I judge for MB I do not take off for that, so yes ther is leeway. The inner fender wells would also be body color, that I do take off for. There is a fair amount about this and the trunk interior finsh etc. on the site. MBCA judging does detract for a paint change, but again, that is dependent on what the participant says, and the level of knowledge of a judge (if they look at the plate, see DB 050 (white) and the car is painted DB 568 (signal red)).
The clamps on my (close) to original car on the rear axel has the clamps in black except for the ones that are holding the brake lines, these would be the 9 mm cotter key style clamps with two small rubber rings (same as the two rubber rings on the hood torsion bar).
None of the clamps on the car are yellow cad, they should be silver. These are available though not in exactly the old style. Again lots on the site about his. And please do not use the "yankee clamps" or the ones with the slots cut clean through the band. I just cringe when I see these on a "high level" restoration.
Some other things depend on the level of the competition. If there are truly exceptional cars that are at the factory specs other cars with replacement parts of different type may get marked down. An example might be the current replacement for the dual brake reservoir above the master cylinder. The replacements are more squared off, and at a slight angle. Some other parts, such as a battery are expected to need replacement, but should be replaced with the proper type (a Varta or similar) versus the green top Interstate or an Optima Gel.
Stainless steel exhaust would be another item, and the judging depends on the knowledge level of the judges. In an a varied marque show, it would probably not be an item, but in a full MB show, it would.
Other items would be the spark plug leads, they should all be straight. Plug wires over the injection lines, or under the injection lines??
Lots of stuff, and some dependent on year, running changes, etc.
Good to chat over a couple of pints of ale!