REACTIONARIES!?! REACTIONARIES!!!
Not mean spirited? Speak for yourself, Joe!!!
LOL!!!

In this case I let the reactionaries do the work for me. The notebook refers to the Electra occupants suffering from personal injury and an heavy arrival remains a possible cause.
That's an excellent application of synergy and synthesis, Tim!
Seriously, I don't know if we will ever know for certain whether those messages were the real deal or not, but if we can find the plane there maybe the evidence will bear these things out. That would be a neat outcome in that more of the story might be known.
As to effects of landing on the reef, I've not been there, only seen pictures; if we've had a 747 captain on the ground there I'd have to bow to his judgment. That said, those big tires on the Electra would have been very forgiving - but in all things there are limits. Even short of damaging impacts, the plane could still have easily been bounced around enough there, IMO, to have knocked the occupants about quite a bit. A head impact would not be hard to get out of that landing.
In fact, there might have even been two unrelated events - the 'hard landing' might have merely been rocky, a bit lurching as the plane negotiated the uneven surface - enough to whack someone's noggin; a gear collapse / separation may have occurred after that fact by dropping the gear in a groove or something, then having the airplane weathercock from tidal forces, etc.
Lots of possibilities. One hopes one day we'll see the first-hand evidence of what actually went.