Cats - big, small, wild, and domestic - eat prey (their natural diet) whole, e.g. muscle, organs, bones, skin, and hair. It would be the very best for our captive and companion cats to eat just that - but this option is not always readily available. Solution: we resort to the next best - raw muscle and organ meats.
However, raw meats alone are not "complete" and lack many essential nutrients provided to the cat when eating the whole carcass - predominantly Calcium (Ca).
A whole animal with bones has a Calcium:Phosphorus ratio of 1.1:1. Meat alone has a Ca:P ratio of 1:20. Now, Phosphorus is by nature attracted to Calcium - forming a chemical bond: Calcium phosphate. If meat where to be ingested without the correct amount of bone, the Phosphorus in meat would draw Calcium from the cat's bones - resulting in disease such as bone deformities, limping, muscle failure, fractures ... The same happens in humans. A diet of hamburger and soft drinks is very high in Phosphorus, and people start to suffer from Osteoporosis as a direct result. The cat has a very high requirement for Calcium due to the high level of Phosphorus in her nearly all meat diet. As omnivores, people require less Calcium, if eating a plant based diet. Herbivores require even less Calcium, and all the Calcium they need is contained within the plants they eat, which also contain small amounts Phosphorus - resulting in a proper ratio.
The Ca-P partnership - Calcium phosphate - is what gives bones and teeth their strength and rigidity and is present in a 2:1 ratio in bone. Isolated Calcium in the body is used for proper muscle function, and isolated Phosphorus represents a structural component of cells, including soft tissue. This is why muscle tissue contains such high levels of Phosphorus.

"Is too much Calcium in the diet harmful as well?"

Yes. The most ideal Ca:P ratio in the cat's diet is 1.1:1, but cats can easily tolerate up to 2:1. Any more Calcium in the diet would cause an over-saturation in the body, and can result in Calcium deposits in soft tissue such as Oxalate stones- especially in the presence of Ascorbic acid (vitamin C)

"Can I feed my cat supplemental bones, such as chicken necks, as the source of Calcium in her diet?"

Feeding chicken necks is recommended, if you can't or don't want to feed whole prey. Whole prey, or alternatively chicken necks, are meant as a change from the every day for the cat, giving her something to gnaw on . However, chicken necks can not be used as a Calcium supplement, because the chicken neck itself, with the meat and marrow, already has a perfect Ca:P ratio of 1.1:1 - therefore, the Calcium in the chicken necks would not be able to balance the Phosphorus from the staple meat diet in addition.

"Do I need to add a Calcium supplement, such as bone meal, to my cat food if I use ground, whole chicken or rabbit carcasses which include the bones?"

No. If you are able to grind a whole carcass, and use it as the meat portion in your cat's diet, no supplemental Calcium is needed, because whole prey - which this essentially is - always represents a perfect Ca:P ratio.
Note: When grinding meat with bones, use a whole carcass, such as a whole "frying chicken" or "fryer rabbit", and not only inexpensive "chicken necks with backs". "Chicken necks with backs" have little meat on them, therefore supplying the cat with too much Calcium, resulting in a diet with a non optimal Calcium : Phosphorus ratio.
Also, when using a ground, whole carcass with bones as the meat portion in your cat's diet, you can not use Feline Future's Instincts TC, because it would result in a over-supplementation of Calcium.

"Are powdered eggshells a good Calcium supplement?"

Calcium from egg shells - Calcium Carbonate - is not as readily absorbed by the body and incorporated into bone as are other Calcium supplement compounds like citrate, lactate, or gluconate. Bone meal, of course, being the most natural and readily absorbed source of Calcium for carnivores, as it is bound to protein. Besides, other minerals and trace minerals are found in ideal ratios within bone meal, making it an excellent multi mineral supplement.