Hi and
I wouldn't worry about humidity getting too low (unless it drops less than 20% or so) I keep mine in the 30%-40% range the first 18 days, then increase to 65% or so. Humidity is important because the fluids in the egg have to evaporate for the chick to have air and room to hatch. The biggest issue I had when I started incubating was too much humidity.
On your question about chickens in-breeding: chickens have 78 chromosomes while humans only have 23. There are many more genes at play than with humans, and one could go 15-20 generations of inbreeding without significant health impacts.
When it comes to shrink wrapping the chicks: There is a thin membrane which coast the egg, when this membrane dries out it hardens and locks the chick into the egg. To avoid this, fill the water channels of the incubator and add wet sponges as needed on day 18. DO NOT open the incubator past day 18 until either all the eggs hatch or day 22-ish.
Also, you asked about the vent plugs: I always keep at least one open at all times. The eggs and developing embryos need air flow and oxygen.
Lastly, if you refer to the egg candling charts, at about day 10 is when I remove eggs that are clear and lack any development. If you have any other questions, feel free to ask. Everyone is here to help you get these chicks hatched! Happy Hatching!
I know that is easier said than done but from internally piping to making the external pip can be up to 24 hours. The chick should know what to do when the oxygen becomes low and c02 high. It kicks them into making the external pip.
