The author is not explicit at all. He barely makes the argument that legalized torture in general will degrade the torturers, as well as the institutions of a liberal democracy. He instead refers the reader to other people's arguments. He does focus on the specific case of torture warrants, but even here he is primarily rebutting specific arguments (by Dershowitz and others) that compare them to wiretaps.
No, that's just wrong.
Nope.
No, you are misconstruing him. He is trying to rebut preemptively the moral absolutist argument that even conceding one-off justifications for torture will lead ineluctably to more routine use and dehumanizing and catastrophic effects. He's trying to separate out the short-term effects from the long-term effects by sticking to a clean one-off situation. Although he clearly agrees that the long-term effects of routine torture will be bad (and he assumes of course that it's immoral too), that is not the point of bringing it up in the first 2/3 of his paper. And in the final 1/3 of the paper, he is only referring to legalization. He even rebuts the argument that a failure to legalize torture in exceptional circumstances might lead to routine use (because, for example, that it will remain in the shadows where it can flourish).