Remove the surviving chicks to a different pen away from all ur adult birds, then call your local or state Extension office and get a Necropsy on the dead birds if you still have them
Ducks need more of certain minerals that are not in a layer feed if they don't get them they develop leg problems also all chickens/ducks can have oyster shell to prepare them for laying and also to support healthy bone structure in ducks adding brewers yeast to duck feed Is beneficial
Not Necessarily my Brother in law raised a Cornish X Rock (Cobb 500 are what most industries use now) hen to maturity and she laid eggs but she was also the size of a small turkey :p It will most likely depend on the strength of the bird and if it can survive to adulthood
New breeder of American Serama! In ga I just got a Breeding pair a few months ago and she hatched four babies abt maybe three weeks ago all looking good!