Welcome to BYC and the quail forums!
Sounds like your incubation temps and humidity were correct, at least to get the chicks to hatch and without deformities. Falling over backwards can stem from vitamin deficiencies, usually from the parent birds not getting enough nutrients while laying. In other words, genetic. Or possibly during incubation, the humidity was off enough that they were unable to absorb enough nutrients, although you say the other breed of chicks is ok.
If you have any nutri-drench or any vitamin/electrolytes you can add to the water, this will help greatly. Even some pedialyte, the stuff you give to human babies will work too. You need to get some good vitamins and minerals in these chicks to replace what they are missing. Most feed stores carry this sort of thing for chicks and chickens that you can use.
Were you given these eggs by someone else? If so, I would not order them them again, just in case this is genetic in origin.
Keep us posted!