Welcome, Mr Rational! Solid writing style, sensible conclusions, worth a regular visit.
So how can this notion be applied to the 9/11 truth movement? The following is a common example of special pleading (e.g., How can the govt. continue to conceal the conspiracy if literally thousands of people were involved? Special plead: you don’t understand the concept of compartmentalization within the government).Indeed. No matter how compartmentalized the work was, there would be a hell of a lot of people who would say, "You know, I wonder if those weird ceiling tiles I installed at the WTC had anything to do with the collapse."
This argument is special pleading because it avoids having to answer the real problem, namely that even with compartmentalization there still remain thousands of people who have to remain silent. Lets take the people who set up the demolition charges for instance. If they were only given instructions to set charges and nothing else, then I find it a little odd that not a single one of them has come forward with this information. The notion of compartmentalization fails when we realize that if one link in the chain fails, then the entire conspiracy fails.