If the eggs broke under her and she ate them, you would know. There would be a tell tale wet spot, which would take a long time to dry since she's sitting on it, and even when it dries, it leaves the nest bedding material matted and sticky. Especially if such a large number of eggs broke over a short period of time, there's no way the bedding would look the same after that. So if you didn't find any signs, chances are the eggs didn't break. It is possible that she moved them out of the nest because she deemed them unfit for some reason (hens can carry and move eggs around, sometimes overcoming obstacles along the way we wouldn't think possible!)
If their shells are weak and you're having a hard time getting them enough calcium, try offering crushed eggshells instead of oyster shell. Chickens prefer eggshells, they are more natural and familiar to them. Some chickens will eat oyster shell just fine, others will only tolerate it because there's nothing else, and yet others hate it and will not eat it. But they all like eggshells. So try feeding their shells back to them in addition to the layer feed.
I agree with others about getting her chicks instead of letting her brood twice. How old, and whether she'll accept them, really depends - on her, as well as on the chicks. They need to be young and inactive enough to be able to stay under her for the first few days, which is what hens will do at the beginning with new chicks. So if the chicks are old enough to be rowdy and want to come out often, she may get frustrated and peck them to go back under, causing conflict. Weather also plays a role. If it's cold out, they'll want to stay under her longer, so less conflict, but if they are already a few days old and it's warm out, they'll want to spend longer out from under her. Disagreement like that can jeopardize her accepting them. The hen's personality plays a big role, too. Some hens will adopt chicks of any age, tennis balls, kittens, anything really, yet others will murder day old chicks for no reason at all. If you haven't introduced chicks to this hen before, do it carefully and stay with her for a while to make sure she accepts them, and even if she does, always have a back up plan (brooder ready) if she changes her mind. And be emotionally prepared for murder, because that happens, too. Good luck!
