When big chuncks of coral get washed up onto the reef, where do they originate from?
Underwater video taken by our divers shows that there are significant overhangs/caves in places along the reef slope at depths of less than 100 feet. It seems most reasonable that the big chunks of coral that have been thrown up on to the reef flat in some places are the result of uncommonly humongous storms breaking off the overhangs and throwing them up on to the reef flat. The force that would take is sobering to contemplate but I can't imagine any other source.
Is there a "prevailing direction" they tend to move?
The really bad boys come out of the northwest.
If so, then there may be a sort of "debris trail" down-stream of the island, where anything heavy enough to sink would be deposited. ... Aluminum aircraft fragments would be further downstream than chunks of coral, or engines.
That's kind of been my assumption. I would expect the massive bits - engines, center section, landing gear - to be fairly deep but not far laterally from where the plane went over the edge. There may also be a debris trail of smaller "dense" components - batteries, radios, gear motor, flap motor, cables, etc. - that broke free and tumbled down the slope as the fuselage came apart. (But I'm not convinced that we've seen any such debris in the ROV video.)
Components that have more surface area relative to mass - outer wing panels, empennage, etc. - should be either downstream or thrown up onto the reef flat and shoreline to be scavenged by the locals.
Buoyant components - floor boards, fuel and oil tanks, maybe even the cabin door - might have washed up on the shore or been carried through the main passage into the lagoon to fetch up almost anywhere along the lagoon shore.