The black gills of #1 narrow it down to Psathyrella/Agaricus/Lacrymaria/Panaeolus and a few others. Panaeolus generally are restricted to dung or fertilised grassland and the other two are chunky things. Psathyrella are tricky and even seasoned mycologists quite often struggle with them.
If you had broken the gills of #2 you would have known it was a Milkcap.
The pointers for #3/4 are dark gills+free gills+floppy ring+growing on soil
#5 is difficult because the gills are often deceptively dark for a mushroom that'll actually give you a white spore print.
Thanks for the breakdown of the logic used to come to an id, Nick. Very educational. It also highlights the value of having an expert like yourself on hand, no matter how many books I consult.
My original guesses were:
1. No idea
2. A Russula
3&4 An Agaricus species (|;|)
5. Plums and Custard (despite there being no pines nearby!)