I'm not questioning the purity of hatchery stock. I don't even remember the word hatchery coming up until you mentioned it. I don't bash hatcheries. They provide what they provide and that's where I get most of my parent stock. I've gotten the same breeds from different hatcheries. They look different, not because they are not purebred, but because you have different people selecting the parent breeding stock.
To me, purebred chickens really does not mean much other than they should breed true. When I say pure for a trait, that means they have two copies of the gene, not split for it. If they are pure for the blue egg gene, both genes for that trait should be the blue egg gene, not one blue and one white. That way, they breed true for that trait.
A flock of purebred chickens can quickly lose certain traits unless the breeder constantly reinforced that trait. It comes down to strain a lot more than breed to me. The best example I can think of is from an article I saw recently. A breeder took a flock of chickens and split it into two separate flocks. He then started selectively breeding one flock for large size and the other flock for small size. The same purebred chickens, just selectively bred. I don't knwo how many generations it took, but the average size of one flock was 9 times the size of the other. Both flocks were purebreeds of the same breed and the same original parents. No outside blood was brought in. That to me shows the power of selective breeding.
You have spoken to Dr. Bramwell about these and I have not. When I see "commercial" leghorns I assume these are birds that have been selectively bred by someone that knows what they are breeding for to really enhance the egg laying ability and the feed to egg conversion rate. A standard leghorn, whether from a hatchery or from someone's backyard flock, should lay a lot of nice sized eggs. But they are not very likely to be up to the same standard as commercial leghorns.
So that is where I am coming from in suggesting that you won't get the same quality in the first generation cross of birds of unknown quality as you would get by having someone that knows chickens well enough to be teaching about them in a state university that has access to excellent breeding stock careful select the initial breeding stock, then go through a few generations of careful selective breeding for certain traits.
I'm not a expert on Araucanas. From what I have read, that lethal gene is somehow tied to their rumpless nature. It is a recessive gene so you need both copies to be present to kill the chick. Since the chickens that laid these eggs are not rumpless, I would expect that fatal gene to have been bred out of them, though that is just an assumption.
In any cross between Araucanas and Leghorns you would not have any chicks that have two copies of that gene, so the first generation would not suffer any consequences. But since it is a recessive gene, it is possible that the chick is split for it and could give a copy to offspring. That's a problem with recessive genes. They can be hard to totally get rid of since they can hide under the dominant gene.
Sorry I misread that.. You said Commercial Leghorns.