Just some thoughts.
How do you know the GG was running a float carburettor ? Merlins were fitted with normal carburettors, pressure carburettors and injection carburettors. With all the modifications made to this a/c and its engine, I would not take for granted that anything was standard on this engine.
Re the engine cut(if it had a float carb)......if it was due to a rich cut, in all likelyhood you would have seen black smoke from the exhausts due to the over rich mixture....don't see that on any video or still. If it was running a stock fuel system , 12-16psi doesn't seem a lot of pressure to lift fuel to the carb inlet, especially with the reported extremely high G level experienced. So the engine cut could more likely be due to a brief interuption of fuel supply. Again I would be very sceptical that this a/c ran a stock pump/fuel system...maybe it does. With the amount of HP this engine was making , it would require much higher fuel flows and pressures surely ?
What makes you so sure the RH elevator or its torque tube failed ? If the RH tab was indeed fixed in the streamlined position , then the LH elevator/tab combination was imparting a force through the LH elevator structure to the RH elevator via the torque tubes and bellcrank. You are forgetting Newtons 3rd law of action and reaction. The forces are equal throughout the system but will be in opposite directions. When it fails, it will be at the weakest point, thats for the NTSB to find out !
Re the tail wheel extension..........it seems it was designed to extend under large G which the pilot was required to pull in the event of tailwheel extension failure. Not supprising it extended here in this case. It has also been mentioned that the doors also came off and this was an indication of structural damage to the tail due to flutter/twist/or the tab failure. The a/c was doing close to 500mph, I don't know of any a/c with a gear limiting speed in excess of 500mph ??? If you extend any gear at that speed, the doors are surely gonna come off !!