Regarding the amount of time you have to supply your hen with fertile eggs... that can vary depending on how stubborn the hen is. I had an Orp hen who went broody over the winter, so I left some eggs with her. She seemed to sit there forever, but the eggs failed to hatch for some reason. She had lost a lot of weight, but still wouldn't give up the idea of being a mom. I finally took her into the house and gave her another batch of eggs to hatch. At least this way, I could give her a supply of food and water that she could reach easily from the nest box. Anyway, after multiple tries, she finally succeeded in hatching out only ONE chick. If I were you, I'd probably set some eggs in the 'bator just in case things go wrong and she doesn't succeed in hatching her own chicks. This way, you could at least stick some peepers under her when the time is right. Otherwise, you could end up with a broken-hearted hen who continues to starve herself like mine did.
I'm not sure about placing older chicks with a hen, though... but it might work if she feels like she's been broody for long enough already.