A) Probably not very accurate at all - as noted, some shader results don't get passed to the preview.  Since the preview is a product of the graphics (Sreed, OpenGL, DirectX,. software, etc.), the preview display will be a factor of the support in any of these.  For instance, most graphics card OGL drivers can only simulate lighting from a limited number of lights (4 or 8 mostly).  If you have 10 lights and are using OpenGL, you'll only see lighting from as many as can be handled in the preview.

Remember that the GPU on graphics cards is engineered to be extremely fast at doing one thing - rendering 3D graphics (and 2D, but that's easy) - and has vertex, triangle, and texel buffers.  Rendering is, of course, done on the CPU using system memory.
So, basically, if the preview were to attempt to 'render' the shaders as in a real render, it would use the same process  - procedural UVW mapping pixel by pixel.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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