I see 1 rational pirate and 4 utter morons who have paid so much attention to math that they forgot to actually win.

Remember, this isn't any old pirate crew. This is a crew with a particular set of rules that gives different pirates different powers. There's no "hang the rules!" here, and since it's an artificial problem there's an artificial solution.

D has the power to walk away with all of the gold, and A, B, and C dead. He has no incentive to agree to anything less, because if enough votes fail he'll be in the best possible position. This determines E's default result, which is what he makes decisions based off of. Building out the game tree helps here.