I agree with some of what you say, and I disagree with some.
The book does question some of the motives of the Catholic Church througout history, and with good reason. We all know that there have been times in the history Catholic Church when the humans running it have behaved in a less-than-admirable fashion, both in the more ancient times and in modern times as well. As a Catholic, I try to keep in mind the "human factor" vs. the "God factor" when looking at my religion. The book does not minimize Jesus' role, message or abilities. It speculates about Him being married, which really wouldn't faze me...he was God in human form, and as a human, it would stand to reason that he might find someone he wished to love and marry.
As far as most people being able to tell the difference between truth and fiction...well, I think you give a lot of people a lot of credit. We DISers tend to be an intelligent, insightful group...that is not necessarily how the rest of the world works, and there are folks who will believe that because Dan Brown wrote it, it must be true.