Hi
I'm so very sorry for your losses.
Yellow cheesy gunk is the chicken equivalent of puss which will of course smell bad because it is an infection. Whilst I can't say what the underlying cause may be, the infection will need treatment with antibiotics. Here in the UK that would mean a trip to the vets, but you haven't included your location in your profile, so I don't know what the situation is where you live. Location will also have a bearing on whether wet pox is a possible cause, since you wouldn't get that at this time of year in a cold climate.
Once you have a series of deaths like this, I think it's time to seek professional help, even if it is just to perform a necropsy on your most recently dead bird so that you know what you are dealing with. Place her in a plastic bag and refrigerate until you can find an address for your local lab. Some state facilities do them for free, others make a charge. There is a list somewhere on BYC of laboratories in the USA that do necropsies on chickens, but again that may not be applicable to you, depending on your location. I'm not generally one to suggest people seek veterinary assistance for their chickens, but I think in this case where your flock is being struck down by it, it's time to bite the bullet and speak to a vet
I
'm sorry I can't be of more help but other than recognising your description as being puss, I'm not familiar with a respiratory disease that causes this although I wonder if it may be Coryza as that causes breathing difficulties and bad breath.
Hopefully someone else will come along that has experience of these symptoms and knows of a treatment you can buy over the counter.
I will be following to improve my knowledge and to wish you and your flock well. Please keep us updated on progress, either good or bad but keeping fingers crossed it will be the former. It may help to use some Vetrx on them until you get further more knowledgeable advice, as that is good for respiratory problems I believe.
Best wishes
Barbara