What a sad event. Though I never met Mr. Leeward, I flew into his air ranch a number of times.
Though I have no more information than what I see here, and I am not P 51 pilot, I can say that an elevator trim tab failure would be one of the most scary in flight failures I could dread. Yes, it could very easily completely over power the pilot, and indeed even the flight control system itself.
I once suffered a 5 degree elevator trim tab stop rigging error on a Cessna 206 (one quarter that airspeed of a P 51), and that very small control error created control forces which were at a minimum 50 pounds, where they should have been able to be trimmed out completely. I have total empathy for what Mr. Leeward must have had to manage for those last few seconds.