Another thing; extra sodium is not helpful! Too many additives in their water? Herbs? What, exactly. Do they have plain fresh water to drink? Every substance that somebody on the internet recommends isn't necessary, or helpful.
Mary
It sounds like you are doing everything possible, so sorry for your losses.
Cornishx birds are fragile, very sad little guys. I too have zero experience with high altitude meat bird raising, but much prefer the slower growing meat types rather then these.
Have you fixed the pasty butt issues...