Context cuts both ways
Of course you are right that we need to be concerned with the context in which our laws were created to be good positivists or even good natural lawyers. However, the moral context leaning against torture is not the only one supporting the Geneva Convention and domestic torture legislation. ALL our laws, national and international, are created with the specific understanding that they can be abridged if necessary to defend the very existence of the Nation.
The Constitution speaks of suspension of habeas corpus in times of rebellion only in the powers of the legislative branch, yet President Lincoln was deemed, under the extraordinarily dangerous circumstances of the Civil War, to have powers to suspend the rights of detainees in wartime. The Vienna Convention on treaty interpretation specifically allows violation of treaty norms if violation is required for self defense.
Put more broadly, one might say that the entire institution of war shows that the applicability of law must be understood in a context of protecting the nation first and foremost. In normal times, a people are not permitted to march with automatic weapons, and gun down people from another country. If arrested in that other country, they would be tried and jailed. However, in wartime, the peoples of the Earth accept that the protection of individual countries allows for temporary suspension in the set of laws which normally govern interpersonal conduct.
The nuclear hypothetical, while perhaps unlikely, poses an even more extreme example of danger to the Republic. The hypothetical could be simply phrased as a question as to the applicability of torture laws in the face of unprecedented domestic catastrophe to human lives, the economy, and the environment all in one.
With war, when the normal veil of law is pierced, the Geneva Convention has given us a level of lawfulness to fall back on. This is perfectly sensible, since in most situations, the presence of war does not really spell imminent risk of national doom. In the normal scenario, loss at war will produce a loss of independence perhaps, or serious economic catastrophe. But historically, loss at war has been unlikely to decimate the civilian population.
The nuclear hypothetical, though, poses another layer of catastrophe. And just as the laws of civilian conduct are breached by the realities of war, the laws of war should be breached by the realities of nuclear cataclysm. The same context which reveals a moral abhorrence to torture reveals a general legal norm that preservation of the state rises above any law.
T. More and J. Waldron are correct for pointing out that even this utilitarian game rises to extreme dimensions (with infants, rape, and murder all in play) very quickly. This gives rise to the point that Prof. Yoo made, which Waldron never fully responded to, which convinced T. More that Prof. Yoo should be writing wills in exurbia. What we do in extreme circumstances may be justified by the context of law, but we will still have to live with ourselves. In the case of the United States, that means the ultimate law a president faces is the political process for him and his party and his policies. So while the torture of a suspect's wife may be justified by extreme circumstances by LAW, it might not be justified to the polity.
Trickery? Evasion? Not at all. The point of the torture memo was to explain to the president of the United States what acts of interrogation would lead to criminal prosecution. The extreme measures of torture justified by utilitarianism probably, if the context justified them, not lead to criminal prosecution. But they would lead to popular rebuke and a powerful statement of our collective moral stances. Then again, if the danger was grave enough, the votes might come down the other way (as they did last November, after the opposition party tried to rally the public around their opposition to Prof. Woo's memo).
The last lesson of all this is that we too, in our time and our circumstances, are entitled to have a moral view of the world that differs from the views of the authors of the Geneva Convention 55 years ago. The act of ratification of the Geneva Convention cannot be said to forever bind the United States to one moral view. It is either moral in absolute terms (in which case, the act of ratification is irrelevant, it would have been morally binding for the millennia before the treaty), or it is bound up in the moral views of one place and time, subject to rethininking by future generations who encounter different experiences informing their own imperfect moral views of the world.
As a parting note, in writing this it strikes me as very curious that the same liberal left which refuses to acknowledge a static morality on any issue (marriage, abortion, drugs, sex, taxes, freedom of contract, etc.) has decided that the views of the Senate in the last century on the topic of torture are etched in stone, never to be subjected to re-examination in new lights and circumstances.