I don't know of any way to keep any duck quiet. That's one of the cautions about Call ducks, that their call is very loud. The drakes aren't bad. They do tend to quack a lot more when they get excited, and you can reduce that to some extent by making sure they always have feed and water...
At least most of the down is usually replaced by the juvenile feathers by 8 weeks. It looks like your little girl already has at least most of her juvenile feathers. First-year ducks molt again into their adult plumage, and that typically happens between 4-5 months old. The molt is a gradual...
The gray drakes actually do have a lot of gray when they're in nuptial plumage. It's a beautiful silvery color that is much prettier than the word 'gray' really conveys. Since ducks don't molt into nuptial plumage like drakes do, they never get the silvery-gray feathers.