3) Writing the raw data is faster when including all the sensor area
As reyalp already answered, DNG always contain all RAW data. Active Area is only Meta Data in DNG to tell viewer software which area from RAW to show (area of "valid" pixel).
My feeling is that anything that contains usable image data should be included,
You mean include Dark Frame in DNG Active Area.
There is a separate setting for the default crop area, which we set to the size of the jpeg (but not exactly the same location!)
Already noticed it, bit annoying...
I've modified pallete.png with white background to make it easier to see Dark Frame in DNG.
Here is a direct compare between JPG and DNG with Dark Frame on SD4000.
There is a lot of postprocessing for JPG going on...