The firmware could be flashed to the device in factory and then write-protected by burning a fuse in the circuit.

This is not the case.

I mean, we don't really have any indication that the MacBook camera light is hard wired into anything.

For those who are interested, you can disassemble/image the assembly and see there is no funny business. People have done it. If there is something more complex than a power line, something is afoot.

I have no way of interrogating some GPU firmware.