There are so many variables in the equation about fuel consumption as to produce answers with a margin of error that makes them useless. That there was enough to get to Howland and operate the radio for a few days seems clear (to us, anyway).
The notion that the airplane floated away and sank seems to me to be wishful thinking based on the hope that an intact, recognizable wreck is on the ocean floor just waiting to be found a la Titanic. But I see nothing to indicate it's anything more than wishful thinking, because there is so much more evidence that the plane broke up on the reef, and most of the remains were salvaged by the islanders or sank there. That there might be an identifiable wreck, or big pieces of it, there has been explored with only a few smallish artifacts found that may indeed be Electra parts. The big bits have not turned up. If they are there and big enough to find, the available search methods haven't seen them. There is always the chance that expanding a search or using new techniques might find something, but also a chance that anything remaining is too broken up and buried to find. I have addressed this in another thread.
As for fuel starting fires, it's an interesting idea, but seems unlikely (and is unsupported by the evidence) that they got any fuel out of the airplane -- they don't seem to have gotten much of anything out of the airplane. There has also been much discussion of this. If they got much stuff onto shore, it's not been found. I consider not only that they might have waited too long to try it, but also that given what I have read about walking on the reef flat, it would have been very difficult to carry anything ashore. I don't think I'd want to attempt (or be very good at) carrying an armload of stuff, on the wet reef, in leather-soled street shoes, after surviving a plane crash. Adrenaline and necessity might make it possible, though.
LTM,
Don