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To my mind, your doggedness here does not mark you a "troll." I think you mean your argument earnestly, and I think you make a legitimate, if somewhat misguided, point.
Your question is a slightly more realistic version of the classic moral dilemma of whether to take one innocent life to save hundreds or thousands of others. I understand it's all about hypotheticals, and that the theoretical demonstration cannot be sustained if we challenge the hypothetical predicates.
Perhaps you will regard this as another dodge of your question, but hear me out. Even if one were to believe that torturing the detainee to prevent an atrocity would be a morally defensible thing to do, this still does not get to the more important question of the dilemma's actual operation in legal and political reality. That is because the supposed moral legitimacy of the decision to torture in that instance depends upon the actual veracity of the predicates that the detainee provided truthful and fruitful information that did in fact prevent or mitigate the atrocity.
How can a person, or a system, possibly account for such contingencies without effectively condoning the use of torture in "unjustified" instances where the detainee does not provide truthful or useful information (perhaps even providing false and damaging information instead)? To create an "emergency" exception for situations that are inherently and inevitably always described as emergencies is to allow the exception to envelop the rule, which is essentially what has happened under the sophistic justifications offered by the Bush administration's lawyers.
The only possible way to avoid this is to create clear and unequivocal legal rules that would make torture a crime regardless of context, and then simply allow the potential failsafes of jury nullification, prosecutorial discretion, and perhaps even the doctrine of necessity to withhold punishment of a person who engaged in the illegal acts and did in fact procure intelligence that directly contributed to ameliorating or preventing an atrocity.
This does NOT mean creating a written exception or system of immunity in advance of the act, and due to the immense doubt attendant to any such situation, potential torturers would be heavily discouraged from doing so - as opposed to a scenario in which the potential torturers are told that they are free to do "what they need to do," in which case the opposite incentive exists.
Also, even in those cases where the illegal torture led directly to saved lives, perhaps society would nevertheless insist on criminal punishment - but this is the necessary potential consequence of having a system built on laws and considered procedure as opposed to expediency and vague notions of paternalistic protection and blind trust in authority.