Sorry I misunderstood you about the grit/oyster shells. Glad you figured it out. Her body may just need some time to adjust to laying again after molting. Might want to put some no pick lotion on her head to see if that will help stop your roo from pecking.
Your hen could possibly be molting which would explain the bald spot and maybe the bad egg quality. But molting hens usually stop laying. Most likely the rooster is pecking the feathers off of her head when he mounts her. Also why would you stop feeding them grit?
It could possibly be the heat. Has it been hotter than usual the past week? Also you shouldn't really add oyster shells to their feed. You keep it in a seperate container and let them choose how much they need. Each hen will need a different amount of calcium and that's why you should let them...