... brain damage? Sterile? ..... oh dear.....
There is a thing called "Hybred Vigor" It is WHY some of the old-timers started CREATING chickens.. Cross this breed with that breed, keep what you want.. get rid of the stuff you don't want... and bring back the healthy, vigorous, fertile chickens...
It is Inbreeding and Line breeding that can cause problems in the long run - out-crossing to another "line" is necessary. It is the inbreeding and line breeding that can cause brain damage and sterility, as well as other defects, especially if it is done without keeping meticulous records. Maybe they got it backwards?
Almost all of the breeds we raise today were, at one time, cross breeds, mixes, mutts.. etc.... Man has been mixing them probably since the beginning of mankind...
I am glad you asked other people what they did in your area. That is probably the best way to get information pertaining to your location. Checking the aircells is also a really good method. The only other thing I might have added is the weight method, especially for your first few hatches. Since they need to loose a percentage of weight (moisture) while growing, knowing exactly what they are loosing by weight is more accurate than the aircell. This hatch will tell you if there was anything you didn't catch using the aircell method but so far it looks really good.
I had heard somebody was using humidity at 50% at higher altitudes.. and I also heard that the hatcheries had to add oxygen to their hatching rooms. However if you are hatching out 10,000 chickens they probably need extra air...
It sounds like you got it right so far, and your Mutts will probably hatch much better than those poor line bred, inbred, pure bred chickens