A nice introduction to a complicated topic. Unfortunately, differential diagnosis (my specialty for the final decade of my 30-year career in clinical psychology) is complicated, not just by the way the symptoms of these disorders overlap and interact, but by the biases of the clinicians and the coverage policies of insurance companies.
The main point I would raise--and I'm not sure if this actually contradicts anything you've written here--is that ADHD is best understood as a continuum. It's not like __% of people have it and the rest don't. Everybody struggles with focus under some conditions. For some of us, the problem is so severe and so pervasive that medication is essential to normal functioning. Others can get by as long as they stay physically active, but need meds for jobs involving reading and writing reports or following tight schedules.
The disorder definitely has an upside, though. See my article, "This Beautiful Disorder."