The following questions concern the D435 Device.
a) if the pixel value in the depth-map is (say) 4871, it means that the point represented by that pixel is 4871 millimeters (or 4.871 meters) away from the
RealSense device. Right?
b) With my application, the color (or greyscale) data is of little importance/use; I just want to get the depth-map. Is there a way to configure the stream so that it only outputs the depth-map data ONLY to get the fastest rate/bandwidth? If so, please point me to the relevant API
Yes, you are right about the distance in the depth map.
Here is an API that would be useful for you: