Do you know if there are any later reports which would deal with the later models,either Mustang,Bearcat,Jug..?
Might check;
NASA Technical Reports Server
The sampling rate of the video is 30 frames per second. Thinking the HS isn't showing flutter, but the left HS deflected down could be a simple overload which would arise if the right elevator let go and the left elevator was over deflected as a result. This would still put all kinds of stress on the tab and the results would be the same.
The elevator was the weak point in early dive testing, and with the q's at Reno, the elevator load cycles during the race, the loads increasing on the right elevator due to the tab on the left one, and the higher horsepower, the elevator just gave up in the high g turn.