I know exactly what keying is. You cannot insert an aircraft impact and explosion onto a plate of the WTC through chroma-keying. It's simply not possible.
The aircraft is obscured by the towers at impact. That means the helicopter shot of the towers is the foreground plate. Now you might argue that you could key the sky, except that the lighting, exposure and angle of the sun makes this impossible.
Keying the sky is not as easy as you might think anyway. This shot was taken from a great distance away. The UV haze (which is clearly visible in the shot) causes bleed from the sky onto background objects such as the towers. Thus any attempt to key out the sky would result in partial loss of the buildings.
The aircraft is the background plate. It has to be keyed behind the buildings.
Your entire premise is a farce. If they are prepping plate shots in advance of the event, there's no need to do anything live - they can simply insert pre-prepared footage into the broadcast.
Your problem is there are dozens and dozens of videos of the impact, from a range of sources. Your problem is there are dozens and dozens of still photos of this event.
Your problem is hundreds of thousands of people saw this event WITH THEIR OWN EYES. You cannot explain this away. Your theory is utterly ridiculous.
-Gumboot