1) Egg shells don't work as grit. They work as a calcium source. Egg shells are calcium carbonate which is 40% calcium. Most people use Oyster shells for calcium. OS is also calcium carbonate. Any eggs shells will be calcium carbonate.
I think the confusion for grit comes from the fact that people call crushed oyster shell, 'oyster shell grit'. Oyster shell is not grit. Oyster shell is soluble and gets mushy by the time it hits the gizzard.
Grit is insoluble stone that get lodged in the gizzard to aid in grinding grains and other tough foodstuffs. Insoluble means that intestinal acids take a long time for it to dissolve.
2) They can eat cheese. Many people say they can't because they aren't mammals. Any dairy product will break down to amino acids, fats, etc.. Feeding yogurt will get probiotics in them. I feed whole milk to roosters I'm about to butcher.
3) They can eat pretty much anything you SHOULD eat. Not soda, candy, etc..
4) A complete feed. They will only lay year round the first year regardless of what you feed them. Every chicken, (every bird for that matter) has to molt every year. Feathers are 92% protein, eggs contain 6 grams of protein. They can't grow a new winter coat and make eggs at the same time. They will stop while molting and recovering. They may start up afterwards or wait till after winter solstice when days start getting longer.