The yellow-ish diarrhea and it's being lethargic makes me think of blackhead immediately. Birds with blackhead, or Histomoniasis, appear droopy or sleepy, don't want to eat or be around the other birds, may have head discoloration, tend to have ruffled feathers, and one of the final symptoms are sulfur colored diarrhea. It is deadly and must be treated immediately. It is caused by a protozoan that is carried by cecal worms which are found in the soil and transported by other birds and earthworms. Are they housed near chickens or other poultry? Have you spoken to you agriculture extension office to find out of blackhead has been reported in your area? It could be something else because the congestion really doesn't fit with the rest of the symptoms. If they were my birds I would treat them immediately (the entire flock) for Histomoniasis and then treat them all for cecal worms. I wouldn't do both at the same time because the medications can be tough of their bodies. You should look Histomoniasis meleagridis up and see the symptoms yourself and then decide what you think. Good luck! I hope that it is something less serious than I am thinking.
