Apparently you don't. The allure behind flash trading is to avoid rule 602 in regulation NMS: you are not required to fill the order at the NBBO. You can use dark pools to fill orders.
Yes, but the order would not be filled at the dark pool price without the flash trade. Joe's order would be filled at the NBBO on another exchange (NOT the darkpool) and Joe would pay an extra routing fee.
If Joe wanted to fill the order himself on a darkpool, he would not have asked the exchange to flash his order.
If Joe wanted to fill the order himself on a darkpool, he would not have asked the exchange to flash his order.