1) cD is based on frontal area only for convenience' sake. We could relate cD to plan area, as in aeronautics, or wetted area, or any other reference area we choose. However, those are all much harder to measure or calculate on complex three-dimensional bodies such as cars.
6) "Scaling factor" in aeronautics and aerodynamics refers to the necessity of obtaining the same Re in the flow around a model as the full-size object, as in
this paper written by some researchers working down the street from me. It appears you are conflating "scaling factor" with "fineness ratio," as freebeard suggested. But, since production and concept cars have widely varying fineness ratios this isn't an accurate method of predicting Re for cars based on height or frontal area, as I pointed out in a post above. (It might work for 18-wheelers, which have standardized trailer lengths, typically 53').