"Change the federal law that prohibits it"
"Unnecessary. 'Illegal' is not a synonym for 'impossible'."
True, but you still need to change the law. Because if you do not, whoever commits the torture is guilty of a felony and can be prosecuted for it. If the victim dies, the torturer, whether acting from necessity or not, is guilty of premeditated first degree murder under any state or federal law, and can be prosecuted just about anywhere, for the rest of his life. There are plenty of people who oppose torture so adamantly that if you simply allow government agents to torture on a "necessity" basis, but without giving them legal immunity for doing it, those agents will be exposed to prosecution and blackmail by enemies of torture for the rest of their lives, and if they ever come into the possession of such authorities, there is no law that can protect them.
Massachussetts, for example, could arrest and prosecute a military person passing through the Commonwealth, were Massachussetts state law to prohibit torture and allow prosecution in Massachussetts regardless of where the crime was committed. And if torture were not legal by a superseding US law, the US government would have no jurisdiction at all to remove its servicemen from Massachussetts prisons.
Also, having a law that says one thing, but allowing a whole secret society of torturers to exist, above the law, in government is another thing. A bad one. It sets a clear precedent. If THAT can be justified by a loose and unspecified doctrine of necessity, so can anything else.
The laws need to be clear, and enforced.
So? You do what you gotta do, and take the consequences. That's the only way to limit it to cases of true dire necessity.
...if you simply allow government agents to torture on a "necessity" basis, but without giving them legal immunity for doing it....
Since I reject any notion of legal immunity, this argument is irrelevant.
...allowing a whole secret society of torturers to exist, above the law, in government is another thing...
Who said anything about them being above the law? If somebody believes that it's genuinely necessary to do something illegal, then the subsequent trial and punishment is simply part of the price, just like the risk of disfigurement and death is part of the price of defending the nation in more conventional ways.