Sounds like you need to put the oyster shell in a heavier dish or something they can't knock over. How many hens do you have? I have 20 and I put the grit and oyster shell in gravity cat food feeders, lol. I only have to fill them up about once a year. But I bet yours peck at it once they koick it over.
Yeah, I'd keep an eye on her, watch for those soft shell eggs. The danger is her and the others learning to eat eggs from eating them. Check for eggs often during the day and get them out of there quickly if you can. What do you have in the nests? Straw? Pine flakes?