The pink ones are aluminum oxide grinding stones. They're better for use on metals, castings, welded joints, rivets, and rust. The green ones are silicone carbide grinding stones that are for stone, glass, ceramic, porcelain, gemstones, and non-ferrous metals. The shapes are for convenience. If you can't fit the bit where you need it or get the results you want, you can use a different shape. The round discs look like a set of aluminum oxide grinding wheels (the pink ones), a set of felt polishing wheels (the white ones), and maybe a set of silicone carbide grinding wheels (the greenish black ones at the top) that go on one of the mandrels. More on reddit.com

I think when a bit is gunked up you just throw it away. More on reddit.com

I use a Dremel to grind off bad spots or cut off weird shaped protrusions that I know either won't tumble well or would take forever to grind down in the tumbler. I rarely use the Dremel before running them for at least 1 week in coarse though because I feel like I get a better sense of where it would be useful, since it is rather slow. It works fine for a small number of rocks, but I wouldn't preshape a whole batch. Way to time consuming for that. More on reddit.com

That is a dressing stone, used to renew the surface of the grinding wheels and points if they load up with metal. You can also use it to modify the shape of the wheel or point. Norton makes a large version of that for big grinding wheels. More on reddit.com