Regarding the Dehydration mentioned. The injuries may be Fred was suffering from heat stroke, which he may have died from.
Symptoms of heat stroke include severe headache and mental confusion. Heat stroke could be fatal.
From
Betty's Notebook the “man”(Fred) appears to be confused and was complaining of the heat. Betty noted he complained of his head after he said “let me out of here” .
“Let me out” could imply Amelia was blocking Fred’s way out. The Electra’s hatch set up has Amelia blocking Fred’s access to it in their normal seating.(Unique information supportive of Betty). A possible explanation for Amelia’s “ouch” is Fred may have tried to climb over Amelia to get to the hatch above her and stepped on her or hurt her somehow trying to get out of the hot plane and get some air.
“Where are you going” could imply that Fred tried to go out the back and Amelia asked this because she suspected he could not get out that way. Perhaps the rear port door was blocked because it was facing the waves on the edge of the reef, yet the starboard engine was clear to run. (A NE orientation) The plane may have been in the process of going over and was very close to the edge and “slipping”. An earlier outgoing tide, slope of the reef and prevailing winds seem like they would support an orientation where the plane was facing NE during a process of moving SW. Also the change in location from high to low could explain the “knee deep” statement versus the tide constraint of a higher location during earlier signals.
Graphics in pdf attached. (edit: sorry the elevations show the door on wrong side and text is cut off)
As discussed previously, Amelia may have received a foot or ankle injury at some time based on the parts of a man’s and woman’s shoe found with of one set of bones. That injury could have occurred sometime after the landing. For example from a fall on the reef. From
E.W. Lee's report “The reef is excessively slippery – it is extremely difficult to walk on it unladen – and as a result of the frequent falls” they lost supplies