I can respond to the "single" spacing. Every font has a certain amount of leading built in, to provide the most readable display. For TNR, this is about 20% of the nominal point size. The early versions of Word used 10-pt TNR as the default font, which gave a line spacing of 12 points, equivalent to the 6 lpi which was the default for typewriters. Some fonts require much greater leading to display properly. OTOH, some have entirely unreasonable leading. You can always work around this by using Exactly or Multiple spacing that suits your needs. It does require some trial and error to determine what Exactly amount approximates the "single" spacing, but when you have done that, you can set that (or a multiple of it) for all styles for consistency.
But I agree that being able to predict what the combined Spacing After and Spacing Before will be is generally preferable to having the larger amount prevail.