1. There is footage of the airplanes flying into the building.

This was never disputed and is totally irrelevant to the claim at hand.

1. In hindsight, several engineering organizations that investigated the phenomena, decided that a collapse from the fires started was likely (http://en.wikipedia.org/wiki/Collapse_of_the_World_Trade_Center )

There are several theories that offer explanations for the collapse, but they don't rule out explosives.

1. In order to be a conspiracy, there would have had to be 3a. Someone who planted the explosives in a way to cause an organized collapse. 3b. People who shipped the explosives.

Obviously true and I don't see this relevant to the argument you are trying to make. Why is considering this "visibly insane"?

3c. People on the inside of FEMA and the other investigating organizations who looked into it.

AFAIK when analyzing the collapse of the WTC explosives were never considered in any investigation.

3d. People on the inside of the FBI who swept under the rug the evidence for explosives.

AFAIK there has been lots of evidence for explosives, Just typing in "explosives in wtc" into my search engine yields the following as a top result:

http://911research.wtc7.net/essays/thermite/explosive_residues.html

But of course this would require chemical analysis of dust samples, something that I doubt the average FBI agent is qualified to do. So there is no evidence to be swept under the rug if you don't first look for this evidence.

Again, where is the insanity?

3e. Nobody in the group of 3a-3d who had a change of heart and decided to come clean.

Kurt Sonnenfeld would be one exception: http://en.wikipedia.org/wiki/Kurt_Sonnenfeld

AFAIK, people in government jobs are bound to secrecy. Sibel Edmonds has already said she would speak out if she was subpoenaed and there are possibly others that would do so as well.

Also consider the baserate, how often did people come forward who were in the know about government conspiracies. Just one example would be Operation Northwoods which only came to light decades after when somebody discovered it in the archives.