Anyplace out of the rain and weather is a good spot for the oyster shell. I put mine in a dish that hangs on a wall close to their feeder.
Table scraps I dump on the ground, outside in their run, so that the coop bedding isn't messed up. I will toss things like sunflower seeds and cracked corn in the coop bedding, those won't make the bedding nasty, and it encourages the chickens to stir up the bedding and that makes everything look cleaner.
I actually haven't ever tried fermented feed, but I know lots of people here on BYC have. You can do a search and see what comes up.
I don't feed treats until I have the chicks outside and no longer in my house, but as long as they are fed grit (or have access to dirt) then they can have treats. I would maybe start after they are one week old.