I feed egg shells back to mine with no problems. I will even throw a badly cracked one into the run for them to eat. Mine won't peck at them to break them, but if one is already broken, it's fair game.
Generally, broken eggs come from freezing, getting kicked out of a nest box, or everyone deciding they need to use the same box that day. They have crushed them fighting over the box. Three large hens don't fit in a milk crate very well. There are plenty of boxes, but they like to all use the same one or two boxes each day. Which boxes they are vary from day to day, but as soon as someone picks one, they all want it. Gathering eggs several times a day helps, and also makes for cleaner eggs.
Hopefully some of the tricks you are trying will help. Lost eggs are frustrating, no matter the cause.