The background colour replacement I did was just a work-around, I understand it is not the best choice if you have precise color scheme.
I too add cyan to black areas, but not to text because I am afraid that alignment errors of plates might show other inks aside the black text.

I too believe that CMYK -> RGB -> CMYK internal translations problems should have been corrected some versions back. Sadly this problems cannot be seen in the CMYK preview in DP7.