Probably not much help as I am a first time duck egg incubator myself, but we just had a power outage and my incubator was without power all night. Temp dropped to about 82F (below 30 C) and humidity dropped pretty low as well. I was able to bring the temp and humidity back up and 12 hours later candled the eggs. I am happy to report they all have definite movement and are all alive. We are less than a week from hatch day so we will see if they actually hatch.
I think it depends on a lot of different factors, how long temp and humidity are off and by how much, how far along the eggs are. Some might argue that they are very sensitive and temp and humidity have to be exact and consistent all the time and you have to follow a strict regimen of turning and misting and cooling for the best hatch, but I am now firmly in the camp that these little eggs are more durable than we often give them credit for.
I have heard that if temps are a bit too low, they will still hatch, but might take a day or two longer to hatch. Likewise, if they are a bit too high, they may hatch a little bit sooner. I cannot say from experience if this is true or not, though.
I have never hatched duck eggs before, however as with any other fowl, temps are very important. Definitely follow the link that CTKen has left for you to the Ducks section. Lots of helpful folks there.
Good luck with your hatch and welcome to our roost!