Sorry to hear about your girl. These symptoms you mentioned are very common in many chicken illnesses. Have you checked her for mites? Is she eating/drinking? Poop watery or normal? Full crop?
My very first move would be to check her vent for mites and if you find them...treat those first. If...
We have been able to catch a hen laying an egg and remember which hen lays which eggs and then we number them or write on them with a pencil. Since you are letting her sit I don't think there will be any way to know which are which unless you check the number on the egg after it hatches and band...