It's not possible to unlink these parameters because they are simply different interpretations of the same internal parameter.
Although you can't change the density and density colour independently of the decay colour, you can change the bluesky additive amount and bluesky horizon colour. These provide the same level of flexibility that you would get from a system that only had redsky decay and bluesky additive colour (like Terragen Classic), except you now have the ability to control the combined result more precisely.
Matt