Hi there.
You can still set older eggs, the hatch rate will just most likely be lower with the older ones, that doesn't mean none of them will hatch. I don't recommend staggered hatches unless you have a second bator to use as a hatcher. Mostly for the fact that when you go into lockdown the humidity needs to be highered and that puts the later ones in high humidity when they should still be loosing moisture. Also if they are too spread out you'll have chicks rolling eggs that are more vulnerable to being ruined by shaking and jostling unless you plan to take chicks out as soon as they hatch. I'm not sure what to tell you about temps. You don't have a cellar or someplace a bit cooler they can set in?
Yes, let cooled eggs warm to room temps first and that's what I would do. Put them all in and what hatches hatches. As for assisted hatching I am fully in favor of assisting but only if the baby has made an external pip and has been externally pipped for at least 12 (I prefer 18 unless there's bigger issues) hours first. Seldom will you ever "help" a chick that hasn't pipped on it's own and usually you decrease the chances of a chick that may have hatched by compromising the vascular system between chick and egg.
I don't know if call ducks are any harder on the average, but if our incubation methods are what they need they should be able to hatch w/o interference. I would say if individuals are having to help a high percentage of their birds that they need to adjust something, (most likely humidity) and are looking for excuses as to why they need to help. Call duck mommy's hach out babies w/o helping them. I would say if your temps are spot on and your humidity has been good the first 17 days and highered well enough for hatch that you should have limited problems with the hatch. That's my opinion. Take it for what it's worth, I've never attempted to hatch them. You said "some were partly formed", was there any fully formed?
What do you mean "finished laying eggs?" Are they older ducks?? If the ducks themselves are in advanced age, that in itself can cause many eggs to not develope or develope right. The age of the layers is also a factor when you talk about the hatchability.
