What happens if he just bolts and tries to use the Time-Turner as soon as possible? He gets shot, obviously, or at least shot at; but wizards seem hard to kill by mundane means, bullet wounds usually aren't immediately fatal, and fast-moving targets in close quarters are hard to hit. I think he has a good chance of survival, maybe better than if he does what the legendary dark wizard wants.

On the other hand, if future Harry suddenly appeared, Time-Turned, in the forbidden corridor with serious wounds from Muggle weapons, I can't see that doing anything other than causing a massive stink. The fact that present him and the rest of the students got to the forbidden corridor without encountering a stink may count as evidence that it wouldn't work.