Broken Cheek Bone Wasn't Fixed Right Away - Covered by Insurance?

That depends on what your current complaints are and what your policy language is. Some policies only cover traumatic injuries for a certain period of time after the injury or if they pre-existed the policy. By federal law pre-existing clauses will soon disappear. You are somewhat lucky being in california because the state law has a broader definition of reconstructive surgery. If a lay person looking at you sees a clear defect or abnormality caused by the fracture your insurance has to cover the surgery no matter what the policy states. If they then deny it you can file a complaint with the state department of managed health care who will demand an independent medical review by a qualified board certified physician. If that review is in your favor they have no choice but to cover the surgery.

Displaced fractures or fractures involving all supports of the cheek bone (trimalar complex, Temporozygomaticomaxillary fractures) need to be fixed close to the time of the injury with plates and screws. Failure to do so allows the chewing muscle to pull the bone down resulting in facial asymmetry and an elongated orbit. Correcting this fracture after it healed poorly is very challenging.
No one knows what each insurance would do. It is clear that a lot would depend on your policy and how it is interpreted by the person reading your surgeon's request for permission to operate.  Personally, if you do not have a documented functional disability it is unlikely that they would approve such surgery since it would be regarded as cosmetic in nature.