Hi OP - have only just seen your larest update, apologies.
You can get them to regurgitate, but, until you have practised a little, it is a two person job. Basically one person needs to hold the chicken upside down, grasping the body fairly firmly and clamping the wings to the body, the second person then needs to gently straighten the chicken's neck downwards with one hand and, with the other, massage the crop downwards ie towards the beak. Do this for about 10 seconds or so and then hold the chicken right way up for a few minutes so it can breathe! Repeat this a few times and, hopefully, this will expel the crop contents!
At the moment I would avoid giving them crushed shells as this might irritate their crops which will probably be sore and inflamed.
You mentioned feeling the corn from yesterday, in their crops, - is this chicken corn (ie wheat), or is it maize? Is this what they were having as a diet before you aquired them? The reason I ask is that if they were previously on a softer diet, like layers mash or layers pellets, a sudden change to harder food could be what is causing them digestive issues.
How are they in themselves? Still alert and active without droopy combs?