Is it possible that Fred's "delirium" wasn't actually delirium at all? There has been lots of speculation that Fred had suffered a head injury but it was stated that her navigator "was severely injured" and only alluded to a head injury. Lots of ways to get hurt in a bad landing. Could it be possible that his agitation, abruptness, and storming out of the cockpit were actually just a hot, tired, thirsty, hungry, hurt and frightened navigator, who recognized the full gravity of the situation, trying to reason with a pilot who is playing with the radios under false hope of an imminent rescue rather than grabbing what they could and boot scooting out of there while they still could? Could Amelia, in fact, have been the delirious one?
She had a lot more invested, a lot more at stake, and a lot more to lose if they lost the airplane. She might have been reluctant to let Fred salvage anything from the airframe or engines in case of rescue, even at their own peril. Historically, plane crash survivors who go down in remote areas will tear the fuselage apart to salvage anything useful. Its odd that you found no real evidence of an airplane itself at the beach or the Seven Site. There are a heck of a lot of useful (and portable) things in an airplane, even if they did unload everything that wasn't nailed down in New Guinea. I would be draining oil and gas for fires, tearing seat covers off, ripping the padding out of seats, busting out windows, tearing out wires...The plane isn't going anywhere. Might as well use it to survive.... unless, of course, you have mortgaged your future on that airplane and stand to lose everything if you cant get it recovered intact. Is it possible that Fred, having had enough of pleading with her and enough of her trying to preserve the airplane when it was obvious that rescue wasn't coming, was now telling her in no shortage of words "The bloody plane is sinking. Quit mucking around with the radios and get out!" It might explain how Amelia got a supposedly "delirious" Fred out of the cockpit before the airplane went off the reef and why no equipment from the airplane has been found.
We know he was alive at least for the first few days. If he was off his rocker from a head injury, would Amelia be able or willing to shuttle him back and forth across a slippery reef at night in the dark? What I "hear" from Betty's transcripts is two people arguing on a hot mic, not necessarily someone trying to keep a delirious navigator under control. Its seems more plausible, to me anyway, that Fred was angry, she was panicking, and they were butting heads over what to do.