A couple of pieces of new information may shed some light on what happened. One of the boy's last tweets was "No fuel in Kiritimati, going from New Caledonia to Pago Pago, then direct Hawaii. That's about 21 hours of flying in two days." and a witness report says the plane was flying low and suddenly just dove into the ocean.
Pago Pago direct Hawaii is over 2,600 miles (2,400 nautical miles). As I recall, that's just about maximum range for the Bonanza as they had it configured, so you know they were extremely heavy coming out of Pago. The witness report that they were flying low may mean they were actually staggering along in ground effect (just as Earhart did after the takeoff at Lae). Night makes it worse. Under those conditions you don't need an engine failure or even an engine hiccup to bring the plane down. You're on the ragged edge. The slightest error and the airplane stalls. Too low to recover. End of story.
No fuel at Kiritimati a big surprise? It's almost impossible to find avgas anywhere in the Central Pacific. Everybody out there uses turbine equipment. Did somebody tell them there was avgas at Kiritimati and they only learned otherwise later? Or did they not check before they planned their route? This was a major screw up. Suddenly instead of two 1,400 mile hops from Samoa to Hawaii you're faced with a 2,600 mile nonstop leg. And you choose to make the over-gross takeoff in the dark?