Other P-51 racers are saying the oil canning is normal, and transitory, though I'd think they'd eliminate that from a drag standpoint since they're meticulous on maintaining smoothness. The deflection of the port tab with the strbd in trail caught my eye as an engineer.
Placard is 0.75M, but they've tested to 0.85M where the Mach buffet shakes the scoop loose and causes rivets to pop. The aircraft was unairworthy on return. As such, MD will be 0.75M as opposed to the 0.80M Mach misspeak from earlier. This would mean demonstrating the Reno q's during the dive test would have been more unlikely even using an adjusted VEAS for 5,000 ft instead of SL. The structural engineers would have compensated to 5,000 ft q's, but it wouldn't have been demonstrated.
The conversion to metal from fabric was to limit the PIO on dive recovery at high Mach numbers for reason you've noted.
Currently the quick push, post left roll you noted, would have further increased the loads on the right elevator and that might have been what did it. The pitch change occurs immediately after the left roll. This would assume a natural disturbance to cause the roll.
Loads weren't understood as well in the forty's so the engineers overbiult everything. That's why DC-3's are still in service, and the reason these airplanes are lasting so long. But the tails are getting beaten up from the horsepower increase to 4,000 and they are in direct influence of the vibrations caused by the blade pass frequency, and are getting bombarded by the blade tip vortices.
Going to a single tab will put undue loads on the tab free side, and maybe it all just added up. Or the tab actuator rod just had enough since it was supporting double the load.