[Sean Cash]

I would pay the ISO and if the funds bounce back because the account is closed, then feel free to pay the rep.

No, this is terrible advice. You have no obligation to the "reps," only the entity you have an agreement with or any assignee of the agreement. If the ACH bounces back, set those funds aside in a payable account until the entity provides new payment instructions. Does your ISO agreement allow the entity to assign the commissions? Or is there a survivor clause such as if the entity ceases to exist, can the members/former members still accept payment on the terminated entity's behalf?

At some point, someone will come calling for those commissions so don't pay them to random people just because they used to work for the ISO.