You need to call the closest big city "7 On Your Side" type reporter. Tell your story. Show them the emails. Have the media bring so much heat on these people/insurance companies that they'll be begging for bankruptcy.

And yes I'm serious. ESPECIALLY if everyone involved denies coverage.
Someone must be held responsible for this. Their negligence enabled this low life to do what he did. The notion that these people think that they're just going to walk away from this without you being made whole makes my blood boil.

Obviously I am not a lawyer but here I go. Denial because of an intervening criminal act seems lame. Would the insurance have paid if the shipment just disappeared and no criminal act was determined? The guy who stole the item was working as a contractor for the broker. The item was never delivered and eventually was found to be stolen by the broker's contractor.The emails where the broker said that he is responsible directly link him to the driver. OP's contract is with the broker and the broker never completed the contract. Either his insurance should cover the loss, because that is why you buy insurance, or the bond should cover the loss.
OP hired the broker to do a job, he covered his bases as well as he could by obtaining an insurance certificate with him added as a loss payee and made sure the shipment was bonded. He should not suffer any loss.

The broker is responsible 100%, and although he majorly screwed up by not confirming that the driver's insurance was valid, he should not have to go bankrupt when he has insurance and is bonded to cover a loss.