....or that could be impact damage. 500 feet seems low, but that's what
the corridor allows. 30 degrees of flaps... I speculate that slow
flight was being demonstrated. For some reason, the plane slowed past
the stall point at an angle and entered a spin, then had insufficient
altitude to recover.

I'm no expert, but it seems like one plausible scenario that could fit
the evidence.

I wonder if... what if he was working to avoid busting into Class
Bravo, and the maneuvering to do so resulted in an inadvertent loss of
lift?