Lets bear in mind your location of Palm Springs California, just like Sydney for Denis (mcfoto) it is a very sunny place. The ZD is sensitive to light, which also effect. Thus I imply that not only the DR of white and black can effect but also light intensities.
Two of the features I shot were indoors, although strobed. The other was indeed outdoors in partially sunny to overcast conditions. Most of my shooting was done at iso50 and 100 however I did some random shooting at 200 and 400. While 400 was certainly noisier than a Canon or Olympus, it was still useable for what I needed.
Yes the 645ZD is probably best at iso50 and 100 or so. But if you need something specifically otherwise than maybe it was the wrong purchase for you. Then again maybe you had an earlier version which has been improved in later cameras. That happens, look at the Canon MkIII problems which dwarf anything Mamiya has had to correct.
I am sure your experience has been frustrating but I would not immediately assume that the entire camera design is faulty. Perhaps more than anything else, your dealer is at fault.