I would stop turning 1 day early. About half of mine have hatched day 19 and many do have them hatch early so as a precaution I would raise humidity and stop turning a day early. No harm will come if you stop 1 day earlier and then they do hatch on day 21 so why not. I've had to stop turning several batches of japanese bantam or standard EE eggs on day 16 or 17 and had just as good of hatch rates.
Same temp definitely.
Same humidity so long as you aren't trying a really low humidity method. I found small eggs lose moisture faster. Most find quail eggs and other small eggs do better at a slightly higher humidity and I think that's partially the reason. It's not the breed that makes the difference it's the size of the egg and the amount of extra moisture compared to shell surface area to lose it through. I make extra sure humidity does not drop too low during incubation of japanese bantam, serama, or quail eggs.