Something I do to increase calcium is use liquid calcium gluconate, available at feed stores. The recommended amount (from my vet) is a teaspoon per half cup of water. I add that to their food, actually, since they splash so much water, and it can get on their feathers and be a little sticky. I want more in them not on them.
I also sometimes put a tablespoon of the Ca gluconate in a few tablespoons of mashed peas - their favorite treat - if I am particularly concerned about a duck who is having soft eggs and trouble passing them. Sechs had a prolapse over a year ago - a soft egg got stuck, and she could not pass it, and strained till she prolapsed. She is fine, now, by the way - but it took stitches, and we induced broodiness (by accident, but we kept her in a dark place and there happened to be some eggs nearby, she grabbed them and sat for at least two months). The broodiness really helped, I think.
And based on my own experience with my flock, talking to vets, reading things here, with all due respect, I don't think any of us really know what's going on in its entirety.
One friend, who has raised many ducks over the last decade, tells me that she has seen no correlation between soft eggs and prolapse in her flock. But some have seen a correlation. That's one reason I think we don't have a complete picture.