I assume egg layers are typically either fed back to the flock (chickens love eating chicken) or turned into e.g. dog food, but I kinda doubt they do even that with a disease-cull.
Cornish cross birds are likely the "freakish mutant birds" you're talking about. People bred them to grow as fast as possible, seems wrong to call them freakish since we wanted them that way. Its around 8 weeks old when they can't walk, that's the point where they have to be processed.
I wouldn't recommend feeding chickens back to the flock. Chickens will eat other chickens, but you need to be really careful with regards to the cause of death. I recommend anyone raising chickens also raises hogs, if you do lose a bird the hogs will east it and there aren't many illnesses that can cross between those species.
or mcnuggets, or any sort of processed chicken product where the taste of the chicken is drowned out.