I think you are doing everything right, and I'm sure she will be a very good mother.
I had a hen to the same thing, and she actually 18 eggs under her. I found her pretty early on, so took out a few, and left her with 12, (I was worried that she was to small of a hen for that many eggs) and out...
Why would they be staggered?
A hen lays all her eggs before going broody, and only then starts to set.
Unless other hens did indeed find the nest, and added their eggs (which seems unlikely, all my hidden nests have been from just one hen) they should all hatch bang on time.