Isn't there one freewheel unit per power section in the CBOX? Unless I'm mistaken both freewheel units would need to go kaput to lose output drive from the CBOX.
Agreed but one freewheel failure would give the same loss of power as a single engine failure and a controlled ditching as per the video. The engine with the failed freewheel would accelerate under no load which would explain the noise as well.
I think a driveshaft failure from the Cbox to the MRGB would have resulted in a far more dramatic entry to the water.