Thomas Coupling, hi.
You are one to whom I referred when I pointed out there are some extremely well qualified posters here. However, I have to disagree that there is much evidence that the aircraft struck the roof in a nose down attitude. I accept you used the term 'appears'.
Of course you, and others, could be correct, but there is little if anything to rule out the scenario whereby the aircraft had a largely level body attitude on impact. The reason the aircraft penetrated the roof in the attitude it did, is very likely to have been caused by the presence of a significant structural feature within the building which supported the rear of the fuselage.
My main observation is that any catastrophic failure is traditionally followed by mandated groundings, and type inspections. Have there been any such requirements?