“The only contingency in which you lose is if Clinton, Obama, Gore, or Edwards end up president as an independent or Republican candidate, which seems completely unlikely.”
You also lose big time if the initial nominee isn’t the final nominee, since the initial nominee contract “will be merged into the Democrats to Win contract”, this means that if Clinton is nominated under your arb you will have no position in “Dems to Win contract” and you will be short Obama, Gore et al. Those contracts remain open after the Dems nomination until election results are in and you lose not just if one ends up president but if any one ends up as final nominee for the Dems.