Ok, so with a lot of this stuff in mind I experimented a bit and made a quick material that's supposed to be half plastic and half metal. Thought I would share for people like me who are still wrapping their head around this and maybe it will be useful. And to possibly get feedback from the people who are knowledgeable in…
Right, such a rule is absolutely incorrect. Roughness maps to not need to be "processed" like a normal map or anything close to that. They can be packed into any channel you please (provided your shader can read from that channel, reading from normal.a may be hard coded into your shader but that is a different issue…