Mostly how I conceptualize it currently as well. There is of course research out there on chronic pain, and how the experience of such pain might result in physiological changes (e.g., nociceptor "recruitment") that perpetuates and alters the actual pain experience. Although this may then fall into a mix of the misdiagnosed and lumped-together categories. As it currently stands, it seems to just be a bit of a "catch all," perhaps primarily made up of groups 2 and 3 (either of which can also of course below to group 1).
I also like the points PSYDR made--regardless of what it's called, the treatment is X, Y, and Z. I use the same approach when making recommendations for persisting complaints related to mTBI, and honestly, I wouldn't be surprised if many of the same mechanisms that contribute to the development of fibromyalgia are shared/exhibited by those who develop post-concussion syndrome.