Computing the result with the pending withdrawal, we can easily say that you are already in a positive of around $408k if I am not mistaken.  It is possible that the 5 withdrawals of $500k within a day alerted them of possible money laundering, being 4 of the 5 withdrawal request being processed and released. Not saying you had withdrawn $115k a day prior to that $2.5m withdrawal.  Saying Duelbits scammed you of $500k is an overstatement when they are still in the process of verifying your transactions and checking them for possible money laundering.

It may take some time but if there is no fault from your side, then they will eventually release your fund.  They had released $2m+ to you, $500k is a little amount compared to your previous withdrawal request which has been processed.