I have one quibble. On page 47 of the main body (68 of 89 overall)....

The primary criterion for the decision was an adverse projection of the future. If Hussein was not removed from power, the Bush administration expected the sanctions to crumble and Hussein to rebuild his military and his WMD and ballistic missile programs and eventually obtain a nuclear, biological, or chemical deterrent capability; he would then renew aggression against his neighbors, and actively support transnational terrorist movements. If Hussein was removed, administration officials expected Iraq to use its oil wealth and human resources to develop into a democracy, thus serving as a model and a catalyst for wider change in the Islamic world and lowering the chances of armed conflict in Southwest Asia. Hence the risks of inaction were greater than the risks of action.
Those two alternatives seem to be the opposite fringes of a very wide range of possibilities. While I am certain some people held one or both of those assumptions, I suspect those individuals were far from mainstream thinkers and far from the White House. How do you discern between the rhetoric and what the actual decision making was? As you noted on page 16...

There is little indication that he drew ideas from National Review or Weekly Standard. But the pundits and writers did assist the hard liners inside the administration by preparing the public and hence Congress for military action, making the decision to invade seem feasible and necessary.
While you point out that he likely did not draw ideas from the rhetoric in the media, the first quoted paragraph seems to suggests that such rhetoric did, coincidentally, reflect his actual thought process behind the decision. This is either amazing happenstance or, I fear, the result of attributing too much weight to public statements that were intended to sell an idea.

WMD, sponsoring terrorism, and related justifications seem now (and at the time, for some of us) to clearly have been just excuses to appeal to an American public swept up in post-9/11 hysteria and a perception of a quick and easy "victory" in Afghanistan. Those reasons were certainly the public rationale and I think you did a great job of documenting and presenting them. But was it the private rationale, too? I think it's a stretch to conclude that. But, perhaps there is no way to investigate that without doing a Vulcan mind-meld with Bush.

One alternative: It seemed unlikely in 2002 that most of the Middle East was ever going to significantly progress beyond being a collection of authoritarian, theocratic, or hopelessly corrupt and stagnant regimes that existed to keep their rulers entrenched in power, paid for with petrodollars and foreign aid, and sustained politically by demonizing Israel and the US. That is, unless the US could intervene in a more significant way than doling out foreign aid or selling weapons. Saddam proved a convenient and timely excuse to get both feet in the door. That is not to say that we were expecting to embark on something resembling the second "if" in the first quoted paragraph above, but rather than we were embarking on a long-term endeavor to reshape the Middle East, not by creating some Iraqi change agent, but by deepening our involvement. A friendly regime gives us physical access with which to flex our muscle.