There are various ways that you can push the lighting in non-physical directions. For example, the atmosphere and cloud nodes have "Enviro light tint" which changes how they receive light from their surroundings (which predominantly affects their appearance in the shadows). Or you can do something similar but globally for all volumetrics by editing the Enviro Light's "Colour in atmosphere". There is a similar control for affecting the shadows on surfaces (Enviro Light "Colour on surfaces), but that only works in the Standard Renderer and is ignored when rendering with the Path Tracer (at least right now, v4.4).
Ignoring shadows and other rays from individual objects is possible, either by changing settings on those objects or by setting up Render Layers and controlling them per object group. With atmospheres and clouds you're a bit more limited, as you can't disable shadows without disabling other kinds of secondary rays such as reflections (as of v4.4, but this might change in future).