If I understand you correctly then:
The photo cell wires are 16 guage
Nothing wrong with that as long as the light or lights it controls will not overload it.

The photo cell is wired to the neutral side
That is wrong. Switches are to be installed so that the power is removed from the light. Switches wired on he neutral side will turn off the light but power is still at the light.

All automatic controls should (must) have a manual shutoff located before the automatic control.
Appliances (lights or whatever) that are controlled by an automatic control must have markings (label) saying so.

Circuit breakers are not suitable to substitute for a switch.

Not sure, but I don't think it is allowed to install the photo cell in the panel.

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Rick Cantrell
Columbus GA