Final Results:
25 eggs were collected daily over 25 days. Eggs were stored small end down in egg cartons on my kitchen counter with daytime temp of 70° and nighttime temp of 65°. Eggs were not turned during storage.
Nonfertile Eggs: 1
Early Fail Eggs (day 7-10): 1
Fully Developed but Unpipped: 3
Magical Disappearing Egg: 1
Failure to Thrive after Hatch: 1
Hernias: 1 (unassisted late hatch)
Birth Defects: 0
Breach (Wrong End Pip): 1
Successful and Thriving Hatches: 18
Besides testing for fertility of older eggs, I also chose not to turn the eggs during storage, and turned only twice a day during incubation at 6 am and 6 pm. I also forget to turn the eggs 3-5 times during the incubation. The eggs were allowed to hatch completely unassisted.
Statistically, this hatch was consistent with (if not better than) my 3 previous hatches of eggs from the same 2 ducks that were stored no longer than 7-10 days and turned 3 times per day during storage and incubation.
So, my conclusion is that the eggs can be stored for at least 3 weeks without loss of fertility or reduction in quality, that they do not need to be turned during storage if stored in cartons, and that turning twice a day is quite sufficient.
This was my last hatch for the season, so the next experiments will have to wait.
Next experiments:
1. 35 days of room-temperature stored eggs, each marked by date collected.
2. 35 days of refrigerated eggs, each marked by date collected.
3. 10 days of room-temperature stored eggs turned only once a day.
Next update will be in about 7 days. Let's see how successful the actual hatch will be! 
