I think we have a rather different view of chickens and in particular broody hens.

I'm curios and perhaps you can explain this to me. Why do you, and many others recommend candling eggs? It's not as if you can do anything about those that don't develop.
I agree given the right sort of nest most hens, including bantams can cover a dozen eggs and hatch healthy chicks. Why I mentioned the condition of the nest in my post above where I suggested limiting to six eggs is because often on hard based nest the hen can't control a dozen or more eggs.
I wrote an article about it.
There has been some fascinating studies on egg incubation recently. This was the study that many of the later studies were founded on. It's a bit of a must read for anyone incubating eggs either with a hen, or in an incubator.
The chapter headed Egg Turning, Egg Position, and ventilation is particularly relevant.
http://www.scielo.br/scielo.php?pid=S1516-635X2016000600001&script=sci_arttext
What struck me in this article and those who have carried out further research is the accuracy with which a hen can position her eggs. This of course requires conditions whereby she can control these optimal positions.
Further studies based on the above suggest that the position of the egg and the number of turns the hen gives an egg are critical to the embryo development; the first week was given in the article above as a critical stage and later studies have narrowed this down to the first three days.
In my article there is a picture off a hen trying to control her body position relative to her eggs position on a hard based nest.
In part my point is, messing about with the eggs, slipping other eggs under a broody who may already have positioned one set, taking them out and handling them, candling them, exposing them to hostile environments et etc, may not be such a great idea.