I'm not sure where you live, but if it's in the northern hemisphere the heat of summer can cause stress that changes egg laying habits. While ducks won't lay 365 days per year, for your breed it is common to get an egg a day, especially for the first 6-12 months. You are right to be concerned...
Yeah, I would agree with @KaleIAm , they can hold it but within certain limits (up to 3 hours or so, I'd say).
Also, since they lay and poop from the same hole and when broody they can go a day without pooping and then get off the nest and let it all out at once, I'm really think that ducks...
By 6 weeks you can move to 2-3 feedings per day, as much as they finish off in 15 minutes. If you are especially worried or a slower growing breed you can wait until 8 weeks.
That's tough, I'm in no way an expert on the topic but it sounds like you have done everything in your power to give them a safe and healthy environment. If this behavior continues past a day or two I would consider culling.
Looks normal to me. I don't have any research to back this up, but in the heat I wonder if the nest is insulating in the opposite sense and actually keeps the eggs cooler. Would be something interesting to experiment with.
A broken egg can be a lot of things. Ours ended up about 100 yards from the nest. I was out there at the time and heard a commotion, the broody duck and a drake were going at it so I thought he broke the egg (it was completely eaten out). After separating her though she ate another egg, so I'm...
We have 14 ducklings, 4 we hatched in an incubator 5 weeks ago, 4 more wet bought (new breed) 4 weeks ago, and 6 hatched by a broody hen 1 week ago. So not quite as exhausting as yours, but it keeps you busy!!
I just had a broody duck hatch out 6 ducklings last week. So much easier than an incubator and brooder. Make sure they have access to water and food at night the first few weeks (we take away water and food from adults at night to keep the house cleaner). I did separate ours from the flock...
I'm not familiar with Blue Swedish, but drakes don't get adult plumage until at least 15 weeks, I forget the exact timing. At this age the best way to sex the duck is by the quack. If it quacks it's a duck, if it just rasps (quieter, almost sounds hoarse, definitely not a quack) then it's a drake.