The list of diseases parasites can cause is extensive and varied. If they're never been wormed, they probably need it; if you have a body you can send off to a vet or lab for testing, might well be worth it.
'Wry neck' is a symptom of various diseases, some caused by things such as heavy metal toxicity, others by vitamin deficiency, others still by parasite cysts on nerves... Viruses such as Marek's can also cause neck twisting. So can bacterial overload.
Unfortunately all the symptoms you describe can fit many different causes.
Has anything changed in their water or feed or environment within the last few weeks? Even months back can still be relevant, even years back. Many things are cumulative. Even if nothing has obviously changed it doesn't mean it hasn't changed nonetheless, i.e. well or bore waters should be semi-regularly tested. Feedstuffs vary in quality despite attempts to regulate it.
Sounds like you're describing cyanosis, basically heart failure, showing in their combs. That is obviously quite serious and generally fatal. It's common enough for commercial layers or meat birds kept on commercial diets to die of cardiovascular or digestive system diseases or cancers; generally it's liver failure if it's digestive system-based; once they start dying though, or showing symptoms, the problem is usually too far advanced to remedy.
To me it sounds like maybe a mass poisoning case, but there's too many possibilities for me to guess in any helpful way. A lab test would be ideal, failing that, an autopsy you do yourself, at home. There's all sorts of charts to help one identify disease symptoms in organs etc.
I would give them all freshly minced raw garlic in their food, at least a clove per bird, and keep it up for at least a week, and try to find out what it is, which may well take professional help.
Cold pressed extra virgin olive oil can help, basically it acts like aspirin for the heart and restores flexibility to all mucosal and cardiovascular walls, including in the capillaries. One easy way to administer it is on wholemeal bread, preferably with a good helping of cayenne pepper as well; the cayenne, garlic, and olive oil all act as potent anti-inflammatories which will help ease the heart congestion and restore circulation, while the cayenne and garlic, the former though caustic properties and the latter through both volatile oils and sulfur levels, will help launch an assault against internal parasites, bacteria, viruses, etc. Don't worry about overdoing it, one tablespoon of cayenne per 5 hens is fine. They not only can't taste hot things the way we can, they have a vast appetite for them. As much olive oil as they want to consume is also fine, even if it gives them temporary diarrhea, it'll just help shift pathogenic and parasitic overloads.
What are their poops like? It's possible you've got classical blackhead symptoms there, in the darkening combs. Most modern blackhead cases don't show the classical blackening faces but some still do. But, as I said, there's a LOT of things that this could be. Clearly they need help asap as you said.
Best wishes.
