I don't know what your true issue is, and I am sorry for your loss.
Thought I might suggest switching to a feed with more protein. 16% that is in layer is the bare minimum and you might be diminishing that further by feeding fruits and vegetables. 4% calcium that is in layer is too much and can cause kidney damage as well as stunt growth in those who aren't laying such as molting hens, growing pullets, chicks, and roosters. Many have said the life span of all chickens including roosters was lengthened when the quit feeding layer as the sole ration.
For my mixed age and gender flock I use flock raiser with 20% protein. And offer oyster shell on the side for layers since it has only 1% calcium. The extra protein supports growth and muscle development as well as feather growth since that's what they are made of. Sounds like your mixed age and gender flock could benefit. Then I only have to buy one feed for everybody and not worry about someone eating the wrong feed.
How far are your 18 week old chicks quarantined? Do you wash hands and change shoes, service the original residents first to avoid contamination?
Also it is OK to keep your whole flock on grower or raiser permanently (assuming the protein is about 18-22% NEVER less than 16%) as long as you offer the OS on the side. Your flock is large enough it might be worth looking into fermented feed. Check the link in my signature line. I am happy with the results!
I agree, ventilation is your problem if you are getting frost bite... There is way too much moisture in the air! Without which frostbite isn't too much of an issue.... according to the peeps who live in colder regions and don't provide heat in their coops in major sub zero temperatures.
I never saw your poop/coop pics yet. To add them you use that little button that looks like little mountains with a sun or moon in the tool bar....