I wish there was a simple concept to explain this phenomenon: the appearance of widespread unified action (a "conspiracy" in the literal sense of the word), but only because the effects of doing X manifest themselves the same way in different places/people, often for biological reasons but more broadly for structural ones.

I guess you could call it something like, "system-limited emergence," in the sense that different systems can have similar outputs if they are structured the same way.

In other words, the idea is that differing groups of people don't see elves because they are all accessing some hidden reality full of elves, but rather because the drug induces the same reaction in a human body, no matter its location.

This maybe seems obvious for mushrooms or other substances, but I think the same concept applies to other phenomena too: the spread of ideas, political actions, etc. Or maybe I've just been watching too much Ghost in the Shell.

When you realize that everything "we" see is common hallucination, then it will all make sense. The human mind creates the image of a tree, the eye just takes in the light. Change the eye or the mind and our hallucination changes.

So these mushrooms change the mind in a very specific way, but no more strange than putting on red tinted glasses.

Speaking as someone who has involuntary hallucinations, this is a reality taken for granted by most people. I have very different hallucinations when I am dep[ressed vs when I am manic. And you are on the right track in my opinion that "the drug induces the same reaction in a human body, no matter its location."

Yes, you've discovered archetypes. Go read Jung's Red Book, none of this is new.