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Without reading your thread top to bottom. My understanding of duplex loading was to increase ignition speed under large amount of much slower burning powders.
As far as Black being scaled and used as a main powder charge preference._ Your brass shells like a any muzzle loader using Black require no gap left between its powder and projectile. And that rule applies to both rifle and pistol. Saying that I'm sure a brass cartridge case is governed by the same Rule. Filled near being a soft compressed loading if not a hard compressed loading when Black is the powder of choice. Average duplex charge for quicker ignition on rifle was like 3-5 grs of Black I think. Pistol I'm guessing 1-2 grs if any. As I don't believe larger kernels of black were used in pistol applications similar to what was used in many large or over bore rifle reloading's of now obsolete calibers. Only a few of those large bore calibers still exist today that were often reloaded with all Black and then purposely charged using a duplex priming grade. 45-70-405--40-65-50-70.
If still intending to use Black for what appears to be a little Iver Johnson pictured consider 3-FFF would be my suggestion.
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