With the higher humidity, I posted to you that your duck eggs could come earlier than due - so dont worry about this part.
The advice of watching the membrane is correct. Our ducklings sometimes end up with a dry membrane before they are out of the egg which then stops them from turning inside the egg to complete the unzip.
To the many die-hard breeders, we do the wrong thing and assist. We call it the cesarean hatch !
We will wet our fingers with warm water and moisten the membrane if has hardened.
We will even start to remove the membrane from the duckling if we cant see veins. We will tear the membrane only little by little, but at the first sign of fresh blood, we stop, put the egg back into the bator and wait another hour before trying again. Cause once the hole in the egg is that large - the membrane will dry again and further assistance will be required.
With them hatching the wrong end, we tend to provide more assistance to get them out of the egg.
We have even assisted when only a small pip hole is in the egg, but we feel the duclking should have hatched by now. We do find most of these times the duckling had absorbed the yolk and was more than ready to get out of the egg, but if we didnt assist - then it would have died.