i've found that most vets will perform a necropsy for free. i would think the vet who treated your chicken would do that for you, as a courtesy. i've never been charged for one myself, especially when the vet was the treating doctor. Or, i wonder if there is a vet hospital or some other type of entity (state, federal, or county) that might be able to do one.
i'm sorry for your experience with Ideal. i had my one and only order from them in February, and everyone seems fine. If you had so many problems, they may have a bad breeder supplying their chicks.
i'm not sure a prolapse is a genetic issue. i honestly don't know for sure. My one hen that prolapsed, i bought the eggs from a very reputable person on this board. So hers was probably due to it being her first egg, and something got snagged on the way. i'm relieved it hasn't happened again.
i hope you can get your vet to do a necropsy. Even if she is not a chicken specialist, she should be able to give you some sort of idea as to why she passed.