Fritz,
I found this thread late, but have a suggestion about the scanning. You mention not having Photoshop so you also may not have other software that would do HDR (high dynamic range) compositions either. It sounds like the best way to scan the color key proofs would be to scan each proof seperately and then combine those layers in Photoshop or one of the HDR capable programs. Of course if you have the original slides, those would be the best source for new scans done on a capable scanner.
Jim Gunning