@azygous In order to help elucidate completely the matter, both for the OP and for the advancement of the hive-mind, can I ask why you think coccidiosis in a 7mo who has lived there all her life. Wouldn't she already have immunity? Could it be the stress of 'point of lay' added to the mishap with excess calcium intake, could this have suddenly lowered her immunity? Or is it more like a 'all labs are closed, let's cover all bases, amprolium can't do much harm' type of thing?
Then, we both agree it's necrotic enteritis, but based on our different personal experiences, we have different explanations for it. You see it as a consequence of coccidiosis. I recently saw blood tests of a non laying hen after she ate layer for three months, the uric acid was very high, so this stayed with me and I wrote above my understanding of the chain of events that took place.
OP, what (if any) antibiotics you have available?