You make a good point—in this instance, Voldemort is very much difficult to bargain with. However, I don’t agree that that makes the problem impossible. For one thing, there may be solutions which don’t require Voldemort’s cooperation—e.g. silent transfiguration while stalling for time. For another, Harry can still get Voldemort’s cooperation by convincing Voldemort that his current action is not in Voldemort’s interests—for example, that killing Harry will actually bring about the end of the world that Voldemort fears.
You make a good point—in this instance, Voldemort is very much difficult to bargain with. However, I don’t agree that that makes the problem impossible. For one thing, there may be solutions which don’t require Voldemort’s cooperation—e.g. silent transfiguration while stalling for time. For another, Harry can still get Voldemort’s cooperation by convincing Voldemort that his current action is not in Voldemort’s interests—for example, that killing Harry will actually bring about the end of the world that Voldemort fears.