"So how does a system evolve through a set of intermediate stages that hinder the survival of the life form, until the final beneficial structure is acheived?"
You are assuming that the enviroment the life form is in remains static. It doesn't. Some "intermediate stages" help in one environment, and hinder in another.
"If a system is guided by a non-specific force like "survival of the fittest", trial and error, down many paths that end up in complex states, where is the evidence of all the errors?"
Since the eye is soft tissue, the short answer is "gone to dust millenia ago."
"If the answer that the randomness allows the system to try many states that are detrimental to find a new better state (simulated annealing, avoiding local minima) it seems the evolution rate, or presence of wild mutations of complex but useless structures would be all over the place"
Such as the veriform appendix.
Isn't that even less likely that the environment and the system would both change, keeping the system changes always beneficial? More variables required to change.
Since the eye is soft tissue, the short answer is "gone to dust millenia ago."
I'm actually referring to evidence of things evolving right now, or at least during recorded human history.
It seems to me that there would be a multitude of wrong turns that would be observable at any moment, if random changes are driving things.
The roomful of monkeys trying to type out that line of Shakespeare would generate a lot of nonsensical text in the meantime.