[dekhn]

I disagree with your presumptions.

For exmaple, for #3, you say electrical signal. But cells normally use chemical, not electrical signals.

For #1- nope, just need an enzyme or other thing that makes a pigment molecule, and something that can detect a chemical (re-use of an existing protein)

For #2- no nervous system, and cells already have many internal signal processing systems.

For #4- those already existed.

Biology mostly just copies and re-uses systems that already exist. It's still incredible there was a path of mostly random events that led to fully formed eyes, though.