After rereading your original post, I have a couple questions. Did one or several ducks lay the eggs over time, or did you put eggs under the mom at one time? And if you are day 26 now, is that from the time you put then in the incubator, or when she started sitting? And day 1 doesn't start until after the first 24 hours. I like to think day 0, then day 1.
But anyway, I'm wondering about timing, and also how long the pip might have been there before you saw it.
Looking at your pic, once they are active enough to make that big a hole, it will usually proceed to zip, so if it doesn't start zipping soon, you might consider if you plan to assist. Can you see inside the hole and tell if anything looks stuck, like membrane stuck to the baby?