I have heard a rumour unofficially that this is what happened-
FO calls for flaps 1, and CA accidentally hits TOGA. TOGA does not engage until flaps hit 1.
Throttles go to GA power and airplane pitches up. FO, startled, pushes down. CA pulls back and there is a tug of war. CA shears the bolts of his control column.
Upon breaking out of IMC, FO sees water, gets re oriented, and tries to pull out of dive.
CVR “very startling, loud noises, unbelievable, thud which could be JS hitting ceiling.”
They think the thud may have been the JS hitting the ceiling and maybe not wearing the shoulder harness
It took a while for all the acronyms to percolate through my addled grey cells, but when I eventually twigged that "JS hitting the ceiling" was the not a reference to a broken joystick, something went clunk in my head.
If the jumpseater (JS) was not strapped in, maybe he was standing, watching and learning from the more experienced 767 crew, then indeed when the dive started and the 767 went into negative g, he might have found himself on the ceiling.
Assuming that possibility is accepted, what happened next could be the key.
As the plane stabilised in the dive, g forces returned to normal, and the jumpseater came down off the ceiling, but where did he land? With the plane in a steep dive, he would naturally be driven towards the front end.
If he came down (arms and legs flailing around) on top of the FO, maybe right onto his arms and the control yoke, we suddenly have 160 lbs of jumpseater driving the yoke forward regardless of what the Captain is doing next door. That would be when the shear bolts failed.
Failing that, did the JS end up straddled horizontally across the windscreen, the instrument panel, the throttles?
Did he rip the pilots headsets off as his body crashed through?
This would go a long way to explaining any apparent lack of proper response from the pilots.
If it took the three of them five seconds to disentangle his body from the controls, that was five seconds they couldn't afford.
The upside of this scenario is that it eliminates the supposed tug-of war between Captain & FO.
So, question #1; Is it a reasonable possibility the jumpseater was not adequately strapped in?
(remember - all this came from the comment above regarding a suspicious "thud" heard on the CVR)
Nothing to see here; move along please.