I will give a pilot's opinion, mine.Can a 757 fly at 530 mph under 30k feet altitude? NO, too much air drag, air pressure too high, max speed hi 200s near the ground.
And of course, this is where the cruise missile entered, 8 feet up, not 30
That however is not the correct question.
I doubt very much the 757 was moving that fast as it approached the Pentagon. You cited some reasons. I would judge it was less than that by 50 mph or maybe more.
Your cruise missile theory is far worse than the jet airplane which was proven to hit the Pentagon.
Here is what is wrong with your cruise missile theory.
1. No person saw one
2. No source of a missile
3. NTSB did not make that claim, and they investigated
Damage to the Pentagon at entry point is not an explosion, it is an impact
Reconsider your faulty claims.