I don't think that I will have a problem with them ruining the vegetation as I have 320 acres (most of that pasture) to use, as long as I move them often enough. How often do you think they would need to be moved if they were in a 160' square electric netting paddock? Another possibility is that I can let them out around 10 (I believe that they are pretty much done laying by 9) to free range and then round them back up at night. This would definitely cut down on feed costs and allow me to not have to move the paddock as often.
I have no idea how long you can leave them, it will depend on several things your weather being a big one. You can let them out, but without a Shepard I'd want a guardian dog around myself. Other dogs, foxes, even some crazy cats will go for your ducks. Mixing in some ganders couldn't hurt, but then you'd have to deal with them too, and they can be mean, unless you hand raised them.
Is there any type of trailer structure that can have just nest boxes or something for them to lay in? Could they be trained to lay in some type of mobile nest box that I could just move when I move the paddock? Will they come there to lay eggs but not hang out and get it too messy?
Last question first, after they rest sitting the first thing they do is stand up, stretch and poo - so any loitering will result in poo. They do not do this in their nest, but will just a few steps beyond.
First question, not that I know of. We hand raised two ducks because they hatched out away from their mother, one, the dumb one did use the nest I provided her, and she let a skunk take a couple of her eggs. The other would not no matter what I tried, and found her own very hidden spot (the skunk did not find her eggs). Later, when Percy's eggs started hatching, she let me move the eggs to a nest I set up in a large Igloo (dog house) behind chicken wire and under a roof. She continued to set them. This is very very unusual behavior, but she had imprinted on me.
Their instinct is to hide from all commotion, movement, light, noise, etc. I don't see movable, semi open, with other ducks near by in this equation, but you never know.
As far as predators, we have owls that like to get the chickens at night if we don't lock them up. I was thinking that they would go after the ducks if I don't lock them up. Any ideas on how to protect them from owls? maybe a LGD?
Definitely a LGD

and if they have water that would help a lot. The ducks will learn to watch for the owls, you'll still loose some (the dumb ones) but not a total wipe out. They will sound an alarm, the gals will quack their heads off. It is louder than a penned up puppy yelp. Mine tell me when a raccoon is in the yard or in a near by tree sizing them up, and I come running. They are smart, not a peep when our cats are walking by, but let a stray cat look at them and they start honking.