Quite true that some mushroom might kill us while others are highly nutritious. I once heard that there is something like a "general" way of identifying those that are relatively safe for eating.
As for the medicinal value of mushroom, you capped it all when you said science has barely touched the surface when it comes to mushrooms, however, I believe that with time, more research will be done on it.
Due to the way fungi easily drops spores and sprouts, their growth becomes common.
Probably true. Where I live, I've heard there are only one or two really badly poisonous ones, and those can be avoided, but there are still stories in the news every year about people eating the wrong ones and going to the hospital (or worse), so I'd rather not take my chances, and I only eat store-bought ones. Look but don't touch, unless you're a knowledgeable forager!