Damn good points. ADD tells us how little we know about how the rest of the brain interprets these dopamine signals. As you said, ADD medication is essentially amphetamine, which blocks the re-uptake of dopamine: once released, it can’t be cleaned up again. So amphetamine raises the concentration of the dopamine soup.
Why this should increase focus in ADD, rather than cause hyperactivity (as it does in non-ADD humans and rats) is a total mystery.
And we have to always remember that such drugs, and others like antipsychotics and antidepressants, act everywhere in the brain at once. So potentially amphetamines have their main effect in different regions of the brain in ADD and non-ADD people.