IMHO, the volunteering institute will extinct really quickly if TSHTF, labor for food and slavery will take it's place. Same fate upon dumpster divers. The food is not growing there by itself....
I think you are right about the foodbanks and the dumpsters disappearing very fast.
I suspect the food (if it was entering the cities at all) would be diverted to refugee centers and relief organizations.
Mushrooms Demystified is a great text. I have a copy.
It is a bit to involved as a starter text though.
Mushrooming Without Fear by A. Schwab might be a better text to start with, even if some people might find it too simple.
It is still important to go mushroom picking with people who know what they are doing at first though.
Even if you identify correctly it really helps to be shown where to look for different mushrooms and how to find them.