OP, I would recommend reading that book, or at least much of it, to help you answer your question.
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It provides much more detail than a bull will on how torture was actually used. Torture was not used to actually solicit confessions, but to solicit confession readiness. This might seem like a technical difference, but it's significant because confessions were not regarded to have any force of law if they were solicited during torture. You will also find that fairly quickly after Pope Innocent's bull, subsequent Popes regulated torture more strictly (we're talking about in the course of not many years) and eventually put it mostly into disuse. Further, its use was restrained only to those who were gravely suspicious.
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Obviously none of that answers the in principle question of whether or not torture is lawful, but it helps us get a better picture of how it was actually used so that when we look at the issue we know what we are dealing with and what we are not dealing with. And that can help us maintain dispassion, if nothing else.
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In principle, I would have to think that torture is morally licit, although I would at the same time think that the situations where it would be licit would be quite rare. Laws permitting it might not be the most prudent. And I think the Church agrees, as she scaled back the provisions in Pope Innocent's bull rather quickly, and popes prior to Innocent condemned its use.