Ed, like I said - yours is the extreme case. It is possible to have minor cracks without coolant loss. See photo below of one of the 3 cracks in my head, with NO coolant loss. I think you missed the early warning symptom of high pressure when cold.
Now, about the pressure ratings. Section 20-017 of the OM603 engine manual states that the cooling system is to be checked by pressurizing to 1.4 bar. Also, section 20-430 states that the original 1.2 bar radiator cap has been superceded to a 1.4 bar cap, the 1.2 is no longer available. I'm not sure what the "maintenance" manual is. There are 2 chassis manuals, 1 electrical, 1 climate control, and 1 engine manual, and the Technical Data Manual. I have all of the above and also the Mercedes 124 CD-ROM, which has a "maintenance" manual section, which just outlines what items to check at particular intervals, IIRC- not actual service procedures. I'm not pulling this stuff out of thin air, really! It's all in black & white. And yes there are errors and contradictions in some of the manuals, particularly the electrical manual. However if you compare different sections you can determine which part is correct.
Update : I just checked out the 124 CD-ROM to see if I missed something. It appears that the Maintenace Manual has more in there than I recalled (oops). However, it is a generic manual for all Mercedes models, not just the 124 chassis. The section on testing the cooling system does spec 1.0-1.3 bar, but that is for chassis 107, 123, 126, and 201 (up to ~1985). None of those cars used the OM603 engine. The OM603 engine manual does spec 1.4 bar for test purposes. FYI, the newest MB's (210 chassis, I think) have some engines with 2.0 bar (~29 psi) caps! Wow... that's high pressure.
Regards,