They will start setting when they have enough eggs. Its differant for each hen but they should start setting when they get 6-7 eggs in the nest. My last broody hen started setting on 4 eggs. They may not set since their young. I think the eggs will stay good for about 2 weeks before they go bad.
I've used Dumor before. My hens ate it but they didn't like it. They loved Southern states laying mash but I can't get since the SS around here closed. Right now I'm feeding them Bartletts "Complete Egg Ration" and they love it.