Hello Paul,
According to my research there are about 12 different standard bolt circles used in the auto industry. Almost every passenger thing from Mercedes since the war is a 112 mm bolt circle but, as you correctly point out, the 600 uses a much larger circle with a 2 mm larger lug bolt.
Note that I said "almost". The exception to the Mercedes passenger rule is the G wagon which uses a 130 mm bolt circle. Whether Mercedes might have decided to use that for commonality I cannot yet say.
Luis is trying to get me a spare 600 wheel so I can measure out the particulars myself, but he's been very busy and I'll have to wait my turn.
Regarding an adapter plate, it seems that the load rating of a wheel is directly related to the size of the bolt circle. Using a wheel with a smaller bolt circle would reduce the load carrying ability of the wheel. The alloy wheels with the 112 mm bolt circle have a standard weight rating of just over 1500 pounds which would be fine for a 6.3 or 6.9 but would already be at their limit on a 600 just parked in the driveway.
Thanks for getting involved Paul! We will figure this out yet.