If the MGB seized, would the blades have kept spinning and auto-rotating to the water with the gearbox and everything shown, NOT spinning?
I imagine three possibilities:
Transmission internally damaged, seized and the whole thing spinning with the main rotor as it autorotated, or,
Transmission not really internally damaged, but input driveshaft damaged/jammed, and preventing normal transmission rotation, so the whole thing spinning with the main rotor, or,
Everything free to turn, but recall the gear ratio inside the transmission now working adversely, so some internal friction, and no torque resistance from the airframe anymore, so the whole thing spinning more or less together.
The apparent near symmetry fore/aft of what separated from the fuselage makes me think that it could find an inertial balance to rotate closely to the mast axis.