Well first things first - yes, DE shouldn't be used on the bird as they'll breathe it in. It should be used in the dust baths stirred in, etc. But the bigger question is - why did you put it on her? Did you find or suspect parasites?
As for respiratory and the weather, there was actually sneezing. It could be that the temperate weather and the rain combined to make mildew, particularly if they spend days under the coop (where things are undoubtedly likely to be more stuff) so it's possible. But that wasn't what killed her - it was a lack of food and water over a three day period, well not enough to allow her to heat and give her fuel to fight an illness if there was one.
What sort of crumbles, the food? Layer, grower, or another? And how much DE did you give her in the feed? ANd was it food grade specifically? INcidentally, never give DE to a bird that you suspect is ill -withdraw it.
Her droppings could either have indicated a lack of food, or more likely her organs shutting down (the mostly white) and just mostly fluids.
And on coccidiosis, only the most advanced cases have bloody stools. Unfortunately it's a very common misconception that bloody stools have to be present for there to be coccidiosis. Sometimes you can necropsy a bird and find the blood in their ceca, unpassed, if they died of coccidiosis. I'm just confused why the duramycin was started and then stopped (a no-no for any antibiotic).
What happened with the other bird? I feel that's going to be tied closely to this. Mucus can often be confused for intestinal lining, and a few days of rain would certainly cause there to be coccidiosis if there was any chance of it - say a new species the birds hadn't formerly been introduced to.
What is the bedding in their night cages?
Have the birds ever been wormed? Did you finish the course of Sulmet?
First thing you're going to need to do is give your whole flock a four day treatment with yogurt and vitamins, or another probiotic and vitamins. YOu could use the regular water-treatment vitamin/mineral package for poultry. Alternately, since you're going to feed yogurt anyway, I'd make a one-time cleansing mash and then the other three days make a regular bacteria-replenishing mash.
CLeansing mash (this is per every six birds)
1/8th teaspoon of molasses
1 jar of baby food applesauce
3 heaping teaspoons of yogurt
1/2 dropper of polyvisol baby vitamins without iron (vitamin section, drugstores) unless you do water vitamins
1 teaspoon of honey
a handful or more of their crumbles.
Just enough hot water to dissolve the honey a little.
Put the honey and molasses in first, use the tiny bit of hot water to make it liquid, then add the rest of the wet ingredients - stir well. Add the crumbles, stir well, and let the liquid absorb.
If the crumbles are still hard, add a little moderately hot water - just a tiny b it and stir. You want it not to be wet and sticky, but kind of fluffy. Serve that to your girls making sure every one gets some.
Replenishing mash:
Same as above, minus the applesauce and molasses.
The molasses flushes the system of toxins and other things. The applesauce pectin cleanses the digestive tract and also makes the good bacteria happy, acting as a "prebiotic" for the "probiotics". The honey is for taste. The crumbles are just a base for nutrition and to hold the ingredients together. The yogurt is living bacteria of the same sort that colonize and protect the digestive tract of your birds, allowing them to use their nutrition better.
Do this sort of mash any time the conditions of their environment are muddy, wet, etc - or during any time of respiratory illness (as the sinuses drain into the digestive tract through the roof of the mouth), or just the replenishing mash daily during intense stress, or weekly for a health tonic.
It's hard to tell what's going on but I suspect both birds have related issues. It's too coincidental to lose two that short in distance apart.
Also if you haven't wormed them, since they're free ranging I would consider a worming with Wazine17 (piperazine 17%, 1 ounce per gallon of water for only one day). Then in a month reworm them all with something like Fenbendazole (SafeGuard paste, 1 bb to eraser sized piece in the beak one day only - then twice annually). Since they're free ranging they have a lot of access to parasite carriers. It could be that the sloughing you saw was actually due to inflammation o fthe digestive tract due to parasites. Using FOOD GRADE DE in the coop under the bedding can help.
The cardinal rule with DE is that if you see it, you're using too much or it's not stirred in enough. The rate for it in the feed is 2% of total feed weight (2 pounds per 100 pounds of feed). Stirred in very well. Never use it to dust the bird as it still is a very sharp bit of fossil (which is why it's said to nick parasites). It's more sharp and harmful than other particulates which is why you blend it in to make it safe for our birds. Also make sure that the areas where they walk and run are sandy if they're confined there, say in a 'run' situation. Course sand drains and disallows the types of conditions that can cause cocci blooms, or excess bacteria and parasites as bacteria and parasites can't handle its dryness. Also in your coop, use pine dry horse style shavings as the bedding rather than hay. You can pour them into a big box and fluff, the dust will fall to the bottom. Tak ethe shavings on top.
Dryness is key to preventing disease.
Without a necropsy, these are the best suggestions that can be given. I would also look up your local state ag college and extensions and gather information now for necropsies in case you lose another.
