I've heard about something called 'stick free neutral point'. If the CofG was aft and fuel was being used from the wings, could the rearward moment arm have contributed to this condition?
The racers have played around with ballast, they know their cg vs consumables loading chart, and they set the cg to what they can handle throughout the race. Leeward had GG flying for a few years so wouldn't think he'd be pushing that on a Friday, though anything's possible. He did demonstrate he could handle the airplane for 3 laps, so he felt comfortable enough to continue.
The problem though is that he required a lot of trim to get the stick force managable. The one trim tab did double duty and is a significant departure from the original design. They would have tested this configuration to Vdive at altitude, but the dynamic pressure is a lot lower up high as opposed to the 5,100 ft race altitude, and the test would have been done at 1 g.
In terms of stick-free stability, I've included a simple pdf explanation without the eye glossing equations.
http://www.flightlab.net/Flightlab.....c%232BA158.pdf
Intend to go back to the video to visualize a new failure sequence with the starboard elevator shearing and going stick free on that side. This would assume turbulence causing the left roll disturbance and top rudder and stick forward and to the right as recovery inputs. They're briefed, as if Leeward wouldn't already know, to fly those inputs when they find themselves even slightly inverted.
In the turn, the shock wave on the wing moves the N.P. aft and increases stability. That shock induced increase in stability goes away under 1 g.