Ignore the penthouse collapse at your own peril, as it is in fact crucial.
See the footage of the event:
[qimg]http://i11.tinypic.com/6ll7l86.gif[/qimg]
Source video:
http://www.youtube.com/watch?v=Y5_e5yROoqY
Note the shockwave/ripple that travels down the building directly underneath the penthouse as it falls. When this ripple reaches the ground, the building collapses, core first, with a slight eastward lean.
Also note that the penthouse was directly above transfer trusses 1 and 2:
[qimg]http://i53.photobucket.com/albums/g73/chipmunk_stew/wtc7_layout_basic.jpg[/qimg]
[qimg]http://i53.photobucket.com/albums/g73/chipmunk_stew/wtc7_damage_est.jpg[/qimg]
The most logical explanation is that the penthouse debris destroyed or damaged those transfer trusses and started global collapse.