I don't think it's true that poisonous mushrooms can contaminate edible mushrooms if they rub up against each other in the same bag. [The mushroom expert] I consult has never mentioned that, and he's been very clear that touching a poisonous mushroom isn't going to hurt you. So I imagine that if you can touch a poisonous mushroom, then wash your hands and be ok (the poisons are NOT absorbed through the skin!), it would also be ok to wash the edible mushroom off and have it be ok. (On the subject of washing...I always wash my mushrooms, even though everyone says not to. Alton Brown proved to my satisfaction that washing a mushroom doesn't add a substantial amount of water to the mushroom itself. He did a whole experiment with numerous batches of mushrooms, various washing styles, and digital scales. I'm convinced.)
The mushroom spores may or may not have been released, depending on what stage you pick the mushroom at. One of the steps in mushroom i.d. is taking a spore print, since similar looking mushrooms can have different colored spores. So picking a young (i.e., delicious, tender) mushroom could mean taking the spores with you. Still, each mushroom has so many spores that even leaving one in a clump behind translates to millions of spores.
It sounds like that bracket fungus was an artist's polypore (Ganaderma applanatum). They're perennial (lots of mushrooms aren't) and most people don't worry about removing them from the trees, probably for a few reasons: 1) they're wood decomposers (ultimately killing their host) and 2) they'll grow back from the mycelium remaining under the bark. (Even annual mushrooms often grow back from the same mycelium year after year, although the mushroom itself only lasts for a few weeks.) I bet the woman didn't know how old the mushroom was (I don't!); funny how so many people don't like to say, "I don't know." [Actually, it was Steve whom I asked...]
The whole thing about foraging in the park is that it's pretty much not allowed. Steve had that publicized arrest, so I think the rangers cut him some slack, but when one forages in the parks it is best to be covert. There's a fine for removing ANY plant material from the park, up to $1000. I'm pretty sure that wouldn't apply if one were caught picking the oh-so-invasive knotweed. Try to pin the parks people down about berries and nuts, and they won't answer. In the state and national parks the rule is one gallon per person per day for fruits and nuts. My personal philosophy is that gathering fruits and nuts is ok, as long as you leave some to keep the reproductive cycle going. In a place like a park where you pick amelanchier berries I think it's fine to take as many as you'll use, since 1) the berries have an excellent germination rate so even if only a few are left, they'll probably sprout, and 2) even if they do sprout, some city gardener is going to come along and weed them out.
I wouldn't call Sassafras invasive. I love the tree; it's beautiful and useful. Filé for gumbo from the dried fresh leaves and tea from the young roots or root bark. The the tea is rumored to be carcinogenic but I'm not convinced that an occasional glass of iced sassafras tea is going to hurt one. And it's SO delicious, unlike any other flavor. Why feel bad about pulling up a handful of 3" tall seedlings (actually suckers)?
The whole thing about the persimmon tree is a little weird, not because of the fruit, which I'd have no qualms about picking up off the ground, but because of the example it sets. I trust myself to be careful of young saplings, but leading a group of 30 people into the growing area seems irresponsible. And why do it if there's another tree, unfenced, nearby?
I'm a fence leaper, too...usually. It's kind of a do as I say, not as I do thing. I think botanists can be trusted to tread carefully and respect the plants. The general public? Not so much.