I've watched this thread for a bit, and have come to the same conclusion as Jeff. IF the Electra were capabile of flying, and IF there was enough fuel, McKean is about 70 nm, and Canton is about 200 nm away. So, thinking that Fred was injured, but able to get a star sight to fix their location, he "may' have been able to plot a course to Mckean or even Canton. Fuel? who knows. McKean would be 1/2 hour +-, and Canton would be 1.3 +- @150 mph. They would have to have known that if they DID get airbourne, and ran out of fuel, they would be no better off than they were on Niku. If they made it to McKean or Canton, were they going to be any better off? Who's to say.
We know that there were radio signals from the plane within 36 hours previous to the search flights over Niku by the Colorado planes. So, within that 36 hour window, this pretty large aluminum airplane went missing. As Ric has said, the tide heights were increasing, and probably too, the wave heights. So, I'm guessing the question is, Did AE and FN fly off the reef to ---whereever?, Or, did the increasing tides and wave action take the Electra off the reef?
For what its worth, I think that they didnt fly off the reef. I'm thinking that during the original landing, there was some damage to the Electra to prevent it from flying. Possibly a bent port side prop ( because they landed facing north (?), and the water was deeper than on the starboard side), and possibly the port side landing gear strut/and or wheel assembly, caught in a trench in the reef (Nessie?). IF so, running the port engine with a bent prop woud be a disaster worse than they already had. So even if the landing gear were ok and the plane would roll freely, it wasnt going to take off on one engine. There just wasnt enough room. Secondly, IF Nessie was the port landing gear strut, then the Electra wasnt taking off anyway.
The other evidence of artifacts at the 7 site indicate that AE was there. So, how long did it take her to get from the reef landing site, or the original base camp, to the 7 site? My bet is more than 36 hours, considering she would be searching for food, water, and anything else to be able to survive with. With all of this in mind, I still think that the Electra is on the bottom, near the reef ledge.