Hi obsidian! I hatch in an incubator, not under a broody, but I can still help with some of your questions.
Here goes:
1. The eggs do not begin developing until the hen begins sitting on them. So usually the eggs will all hatch at the same time, no matter when they were laid. The exception is if the hen was setting on the eggs and another hen laid fresh eggs in her nest. In that case, the first eggs will hatch first and the later eggs will either hatch later or not at all. Often, once the first batch of eggs hatches, the broody will leave the rest to take care of her babies, and you will lose any that started incubation later.
2. Ummmm.... yes, but only if you remove them from the hen upon hatching and keep them with you 24 hours a day, talking to them, holding them against you, and letting them follow you around the house. Imprinting is not, despite popular ideas about it, an instant event. Imprinting occurs over a course of days and requires 24/7 close contact with the object desired. Frankly, I don't think it's fair to a bird to cause it to imprint on humans--it can never live a natural life and will always be wanting things it can't have (like mating which is not possible between species, lol, and constant companionship which is not practical for humans). BUT, I suspect that what you really want is for the little ones to be sweet and friendly with people, and the best way to accomplish that is simply to spend time with them. If you remove them from the mother and raise them yourself, you can teach them to be very friendly without causing them to imprint on you. I raise all our ducklings in the house at first, and they become very friendly the more they are handled and around people, but we've never had them actually imprint on us, because we're not with them 24/7. I had one small group of ducklings that was raised in a classroom and handled literally non-stop by children from 8 am until 3 pm every day. They were the friendliest ducks I've ever ever met. But even they were not "imprinted" on humans. Still, they were a great example of the difference that frequent handling creates. On the opposite side, I have some ducklings that I raised but that are extremely skittish because for two out of their first four weeks I was not available (I was out of town one week and sick another), so they haven't been handled much and they are by far the least friendly ducks I've ever had.
3. Ducklings can swim almost as soon as they can walk. However, most people don't let them in the pool until they are at least a week old, and then only under close supervision. Until they get true feathers, they are not waterproof and can get waterlogged and drown. Also, they aren't good at finding the way out, and can get chilled and/or drown even if there's a ramp. They are not the smartest crayons in the box. They must have a warm dry place to get dried out after swimming, also. The mother duck will take good care of them, but I still wouldn't give them access to swimming water without human supervision until they are a few weeks old. Too many dangers that mama duck may not be able to prevent.
4. Yes. The other grown-up ducks are likely to not like the ducklings. Mine attack new ducklings, and the mother alone won't be able to protect them adequately. Bonnie won't be able to protect them from much, but I wouldn't worry about the holes in the fence--they won't stray far from her. I would be far more worried about hawks, owls, and jealous older ducks.
5. Good luck and have fun. Ducks have been doing this for a long time without us, so if in doubt let mama figure it out. Still, there are a lot of things we can do to help her, so you're right to be asking questions. Have fun!!
