If you are going to let her incubate eggs you really need to have a brooding area where the other hens can't get to her nest. Unless she is head hen and NO ONE messes with her, the others may kick her out of her nest and lay an egg in it, then she can return. You really don't want non fertile eggs being incubated for up to 21 days. Plus, you want the eggs to hatch at ground level so after the chicks hatch there isn't an issue of her getting them to food and water without dropping out of an elevated nest box.
As far as a hen protecting her chicks, don't worry about it. They turn into the Hen from Hell when they have chicks to protect. None of the other girls would dare mess with her or them. I had a batch of chicks from Meyer that I stuffed under Zorra in 2015. She is a big Black Australorp but was mid way in the pecking order. Soon as she had chicks even the 2 Anconas (#1 & 2) waited until her back was turned and ran as fast as they could to get past her. I've seen her more than double her already large size when she was fending off potential threats to her chicks. Even the Tasmanian Devil would run from that.