I suggest popping the chicken bones into the crockpot with some broth and maybe some brown rice. Let it cook on low for about 24 hours. This will make the bones crumble and your cats will be able to chew through them much easier.
I would be careful with the eggshells because they can be abrassive.
Calcium supplements can be useful and your veterinarian should be able to recommend a supplement. Just be careful on the amount you use because it can deplete other nutrients in the body such as zinc and magnesium.
Bonemeal can contain undesireable heavy metal contaminents such as lead so please be careful.
Seconding what AC said. Cooking doesn't make bones "crumble" it makes them splinter
. Just try it yourself: take a thin cooked bone and a thin raw bone and try breaking them with your hands. You'll see how hard and brittle the cooked bone is, while the raw bone is softer and more pliable.
If you think eggshells are problematic then I can't imagine why you think cooked bones would be okay. Eggshells are extremely fragile and should pose no issue for a cat.
Also...brown rice? Broth? The primary notion behind raw feeding is to give cats a more species-appropriate diet, to escape all the junk companies put in commercial pet food. So why would you add grains and salt back in to your homemade diet? I can see using broth temporarily as a bribe if a cat isn't eating, but I certainly wouldn't recommend it as a regular feeding practice. And in addition to being extremely
high in salt, many chicken broths can contain things such as: wheat gluten, soy, MSG, corn oil, sugar, onion, and garlic.
As for bone meal, buy bone meal that is human-grade, as AC said. You should see a nice little "certified heavy metal free" or similar sort of label on the bottle. Avoid fertilizer-grade bone meal.