It's not what is in one bite, it's the total grams of calcium she eats all day, and even that is averaged over a few days. If you are mixing the feed or she eats a fair amount of low calcium foods while foraging then Layer may not provide sufficient calcium for egg shells. If she is getting extra calcium from other sources it may be enough. Layer is formulated so that if all they eat is Layer they will get enough calcium for egg shells.
Yes, as someone mentioned the largest size should be about the size of a green pea. I don't know how raw your oyster shell would be. If they are old and dried up they may be OK as is but if they are from a restaurant or oyster bar they are going to have bits of organic matter and maybe other stuff on them. They are probably going to smell pretty strongly. Washing and then baking or just boiling to start with should neutralize that small and stop that organic matter from smelling. After they are boiled you want them dried. If you try to store them wet you may allow mold to grow.
If they eat anything other than Layer I think a separate source of supplemental calcium is a good idea. As long as it is a separate container they can regulate how much they eat.
I agree, my preference is also that they don't start laying so young for the same reason plus they have fewer start-up problems like soft shells if they wait a few weeks. I don't know how to stop them once they start. But that is a question, has she really started laying or was that just a fluke.? Or was that just a start-up problem where she will soon be laying eggs with a good shell? These kinds of things are not easy. I think the best thing to do is observe and see what happens to decide on your next move.