I'm not sure what you mean by "autonomy" here. The scientists guess that the device is reading or writing, but a third possibility is that it is doing both, and is a kind of brain computer interface. In essence you might as well say it is part of the thing there: the human-black box combination has just as much autonomy as normal humans have.

"Suppose the decision maker was infallible. Everyone who smokes dies. Sooner or later people would all stop smoking. And this is where the scenario doesn't work anymore. Because the number of people dying can't go down. So either it must be impossible to convince people – in that case, why try? – or the decision maker becomes fallible, in which case your whole argument breaks apart. You don't smoke and still die."

In real life it does seem impossible to convince people; there are plenty of people who are stubborn two-boxers, and plenty of people stubbornly insisting on smoking in the smoking lesion, like yourself. So nothing in my experience rules out it being impossible to convince everyone because of the box. Nonetheless, if the box is writing people's choices, that does not mean it will permanently be impossible to persuade people. It will be impossible to persuade people who already have the opposite written; but if we succeed in the future in persuading everyone, it will mean that everyone in the future had their dial set to the second position. Nothing says that the proportion of people in the population with the dial set one way or another can't change; it may be being beamed in by aliens, and perhaps you are cooperating with them by trying to persuade people.

"Think about this fact again: no forecaster means there is a fixed percentage of the population who has their box on state 1."

So what. The proportion of the population who will in real life die of cancer is a fixed proportion; everyone who is going to die of cancer, is going to die of it, and everyone who isn't, isn't. That doesn't mean the proportion can't change in the future, either in the real life case, or in the box case.