What is the correct position for one supplicating for the deceased in the graveyard when in doubt about the location of the grave, and does the deceased hear the supplication of the one supplicating for them?

There is no specific place for supplicating for the deceased. It is legislated for anyone who visits the grave of a relative or a Muslim to supplicate for them, whether they know the location of their grave or not, because the One being supplicated to is Allah. If Allah answers the supplication, the deceased benefits, whether the supplication was at their grave or far from it, and and whether the deceased heard it or not. There is no impediment to the deceased hearing the supplication if they are in a place from which they could hear if they were alive.