I am very sorry for your losses. I have had some mysterious losses in my flock since I started keeping chickens ~2 years ago. I did have a veterinarian do a necropsy (post-mortem dissection) of two of my birds and sent the samples to the state lab. Came back as possible Lymphoid Leukosis (some consider this a variant of Mereks). These birds showed weakness/limping/neurologic signs.
The best diagnostic tool for chickens is (unfortunately) the post-mortem exam. And the very best exam is done when the lab gets the entire bird. You should find the phone number(s) addresses and hours of the labs nearest to you, and find out how to get there and whether they accept specimens brought in person (get the protocol for doing this, as there are usually forms to fill out, and perhaps special instructions, especially with regard to preparing a specimen). They can give you prices too. After they pass, chickens start decomposing very quickly due to their high body temps. The best way to preserve a bird for the lab is to get to it ASAP after is passes and wet the bird down in cool water, wrap it in damp newspaper and refrigerate it (freezing can damage tissues). Then call the lab or follow their protocol to get the bird there ASAP. But it's always best to have this info ahead of time so you can get the best answers.
Please note that cocci and coccidia are NOT the same thing. Cocci are round bacteria (staphylococcus is an example of a cocci). Coccidia is a protozoan parasite which causes diarrhea (sometimes bloody), and can cause illness/death in chicks/young chickens. Also, pale combs and blue/purple combs are not the same thing. Pale refers to a lighter shade of red (which as someone mentioned, may not be abnormal) and can be a sign of anemia, or low red blood cell levels. Blue/purple color (cyanotic combs) refer to blood that does not contain enough oxygen, which, as someone mentioned, can be a sign of some respiratory diseases. However, anemic animals can have compromised oxygen-carrying ability.
Hope this all helps. It's hard to have and then lose such wonderful animals and not know why. The more chickens I keep the more I learn about them. I have not been able to find a great resource on chicken health/anatomy/care/disease/treatments. For me, it has been and may always be a trial & error, continual learning experience. Chickens are complex animals, and I think, for me, it's been very challenging to learn about them because they are so different from mammals.
Yikes! Sorry to have rambled! Signing off now... And best of luck to you and figuring out what's going on with your babies!