I believe the NTSB account, but based on this graph, http://twa800.com/lahr/zoom-climb.htm, why did the flight not stall after the nose separated?
In a nutshell, the aircraft had enough energy to climb without exceeding its stall AOA. The expert stated the plane should have "immediately stalled", but the incredible amounts of mass and velocity associated with a plane that large demand a closer analysis to the physics.
When the nose breaks free, lift and weight are out of balance. When an aerobatic plane applies full up elevator, lift and weight are out of balance.
There are two "bookends" to what happens next. The upper is G limits are exceeded and the wings tear off. The lower is stall AOA is exceeded. In between is an attempted loop.
Apparently, the rugged 747 had enough velocity to climb, sans nose, for a while. Really a testament to its design.
Even after stalling, its momentum would have carried still higher, so it is difficult to determine when exactly it stalled.
Factors which determine "stallability" include wing loading, wing configuration, and tail volume. A very high wing loading with a very thin wing (as seen on many supersonic fighter aircraft) will stall much faster. Deploying slats and partial flaps greatly helps a plane tolerate higher angles of attack. Large tail volume also increasingly inhibits higher and higher AOA.
Essentially, it is a race between change in flight path and change in AOA within the G limits of the aircraft.