I don't think they do. AFAIK most of your assertions above are unknowns.

My theory, at least re: psilocybin and the like, is that a parasitic fungus developed neurotransmitter agonists as a way of manipulating its host. The complete cocktail that was needed to make the target species behave in a way that completed the fungus' lifecycle may be lost to time, but certain components of it still exist in the gene pool.

And if you're going to have millions of spores (rather than the two or three offspring that, say, humans have) you can afford to set them up with a wider array of gene expression profiles. So you've got these fungi just trying stuff to see what works--now armed with neurotransmitter agonists as part of that "stuff".

Later on you've got other organisms eating those mushrooms and behaving strangely. If you're a fungal spore, and you're in an animal's digestive tract, "strangely" is probably how you want it to be behaving. You're going to end up in a different place or with different nutrients or in some other situation which is abnormal for your host--you end up with greater habitat diversity.

How specifically that host-behavior-strangeness would lead to a lifecycle that would reinforce the inclusion of psychedelics to the point where P. Cubensis is reliably psychedelic, rather than just the occasional mutant, I have no idea, but when you've got billions of years to play with, it's not hard to accept that some pretty weird animal-fungus interactions came up here and there.