You just said it all. A chicken that's grown hardy from being allowed to be a chicken, exposed to the weather (within reason) and ordinary outside germs, is healthy.
You know the kid whose parents are always running to the faucet to wash off their binky (pacificer) after it's dropped on a clean floor? Those are the kids that grow up with awful allergies. How can you develop immunity to germs if you're never exposed to them?
No added light here. Most days I get eight eggs, somedays twelve. Out of 16 hens, four aren't mature enough to lay yet. Four are still getting their egg factories up and running.
We have added light, but in the morning and not at night. And not that much extra. Maybe 2 hours extra, just so that darned roo can get his groove going by the time I leave for work. I have the weirdest roo on the planet.
My father (Maine farm boy, remembers his dad's plow horses) told me that when it is going to rain all day, the chickens will spend some time preening and oiling their feathers, then will stay out all day. If it is going to rain off and on, they won't bother, but will run back to shelter when it rains and run back outside when it stops. I've been watching, it's true!
My chickens are healthy as well, and in the rainy weather the water pearls up and runs off them like it does on a duck. They don't seem to be very stressed by the weather thus far this year. So, I am just taking everything one day at a time. We did have a brief spell of dry freezing temps and they did absolutely fine through that too. I always make sure they have plenty of clean dry shavings as well though.