#2) Whether ADHD is "real" or not requires a complex discussion about epistemology, etc, as do all mental health diagnoses. What do you mean, "real"? Yes, of course it is real in so far as there are people with various clusters of symptoms that seem to go together, but they are all on a spectrum. Psychiatric diagnoses are inherently unreliable[1] -- in that it can't generally be replicated sufficiently from doctor to doctor with the same patient (or even doctor to same patient at different times) even with that giant bible sitting there we call the DSM.
So yes, there are a variety of states of consciousness that vaguely cluster together that we have decided to call "depression". We know some chemicals that help some of these states -- though it's arguable that many of them are better than placebo (hello, SSRIs, but hey ketamine seems quite promising). Any therapist worth hir salt knows that depressions are different for different people -- different causes different manifestations etc. Yet thanks largely to the pharma industry, we have begun to collectively think there's this very specific thing called "depression" that's due to "brain chemistry" that can be fixed with XYZ and in fact human consciousness is much much much more complicated than that -- from both a psycho-spiritual angle and from a neuroscientific angle.
Lastly, there's no doubt amphetamine usage has been around for a long time, and probably at higher concentrations than today among some population groups (middle-aged women in the 50s for example). But I doubt you can find me any study showing that we've ever seen so many people on high-dose daily amphetamines from a young age through adulthood until the past decade or so.
#2) I agree that ADHD is a complex subject and diagnosing is hard. However, given that something like 80% of people with ADHD show improvement with amphetamines, I'm not sure what the point is of this line of argument. Even if ADHD really is a cluster of similar issues, if they all can be treated the same way, then it doesn't really matter whether it's a single issue or multiple.
I'm not sure why you're trying to draw a comparison with depression. The only real connection here is that they're both psychiatric diagnoses, but beyond that they're not the same at all.
> But I doubt you can find me any study showing that we've ever seen so many people on high-dose daily amphetamines from a young age through adulthood until the past decade or so.
That's completely irrelevant. Why would you want a study looking at the number of people being prescribed? There's nothing you can conclude from the fact that more people are taking it now than before, because the number of people taking it has no relevance to its long-term effects. All that really matters is whether a statistically significant number of people have been taking it, and that's certainly been true.
> In any case, we can keep pretending that stimulants are just good medicine for a vaguely nebulous diagnosis and poison for everyone else
That is a gross misstatement of the position you're arguing against.
> As far as long term effects of taking amphetamines every day, especially starting from childhood...well, we're running a population wide experiment on that now
Amphetamines were discovered over 100 years ago, and have been used for therapeutic purposes since 1935. In fact, it appears that as early as 1937 amphetamines were being tested on children with severe behavioral problems (i.e. what would be finally classified as ADHD). We may be using amphetamines much more often now for therapeutic purposes, but pretending that nobody has been taking amphetamines long-term until recently is wrong.