At 20 weeks old they're still extremely new to laying, so problematic eggs, eggs dropped in the wrong place or wrong time, are all very normal during this period.
I didn't bother with eggshells until this past winter or maybe spring? It definitely helps stretch out the oyster shell (which I still use, but I need to buy a lot less of it now) and I find it encourages birds that were less likely to take oyster shell to at least intake some calcium, as they...
Except they can't go to the fridge for cheese whenever they feel they need more calcium, and if it's high in calcium as you say then they also need to be able to opt out of receiving it. Not sure if the fat content is an issue but not going to pretend I know how to convert nutritional info for...
They should always have a calcium source available, layer feed has calcium to meet an "average" bird's needs. Some birds need more than that.
I don't think you can meet their needs with cheese, and possibly chickens can't digest dairy (though I'm not sure on that).