My last Marans I incubated at 16% and got a 100% hatch rate. The dry air really does help. I did put a damp rag in there at hatch time and I think it was around 40% for hatch - much lower than what I usually aim for. I think these are the first EE eggs that I've hatched, IIRC.
I think part of the problem is I am incubating in my Chinese incubator instead of the Farm Innovators, which is what I used for the Marans I mentioned above. I have to keep the Chinese one wrapped in styrofoam and I did drill ventilation holes into the plastic and the styrofoam approximately corresponding to what the FI has but I think it tends to remain a bit more humid because of the styrofoam. The other thing is that the weather has been nice and cool/warm enough (however you want to look at it) for us to not run air or heat and to keep the windows open, so that is adding to some extra moisture in the air in the house as well. I did have a hygrometer in there but it kept falling in between the turner rows and jamming it up, which it doesn't do in the FI, so I took it out. I think I adjusted the humidity alarm to 20% last year and it hasn't been going off.
My brooder coop is about 2.5'x6' with a 4'x6' run that opens up into the main run, so there is plenty of room for each broody to pick a corner/area and stay out of each others way. I keep forgetting to count the eggs but I think I have about three dozen and that is too many for my girls so hopefully I can get the majority sold quick. I will probably just ask $1 for the mixes to move them along.
I'm sure it was an issue of the baby running to the wrong mama, but usually IME they are not so discriminatory about the babies that soon. Maybe it depends on the mama. With my Japanese I can keep giving her babies over the course of a week and she will accept them all.
I had one of those Chinese bators, the temp sensor went out on day 18! I had to keep a heat lamp over top of it to keep the temperature up. Then I bought 2 Brinsea Octagons. Then I got an FI still air styro to hatch in. Its done well, but I know exactly what you mean about dealing with the fluctuations because of the changes going on outside. The Brinsea's don't seem to care, but the others sure do.
When I checked the broodies again, the frizzle had broken through the barrier that I put up, and was trying to snuggle under the black hen, and doing a lot of pecking in the shavings. I'm 99% sure it was the frizzle that killed the chick, so I just removed her and put her back in with her group. She can go back in the main coop and sit on nothing if she wants, or she can snap out of being broody. I don't care. I'm pretty sure the black hen will do fine on her own. I've only let 2 hens co-brood once before, and after about a week, one kinda took over, and all the chicks followed her, and the other just went on about her business. I doubt I will let it happen again.
So how's your hatch coming along? Today is day 14 for my big set of Seramas and SDW OEGBs.