How precious. You have some smart ducks. As for immature lungs that I don’t know could be possible but only a vet could tell you for sure. Hopefully it’s something she’ll either grow out of or it’s just her and just normal for her. I can truly understand your being concerned but your doing all you can to keep them healthy and happy so just enjoy them. I’d def give them a waterer they can dunk their heads into anytime they want especially when eating. And your feeder maybe getting to small since it’s for chicks and ducklings have wider bills. You can use a plastic cool whip bowl with a lid cut a hole large enough they can stick their heads into and enlarge as they grow for their waterer.