By the way, that nut is an electrical lock nut, which should be regular pipe thread. A pair of pliers should be able to grab it, but its difficult to figure out from here if there is enough room to fit in that area. You will have to be careful not to damage the basin, but you MAY be able to jam something like a large screwdriver in there to keep the nut from turning and try turning the valve body again, but as Jimb said, it may not be removable from the top up, as the valve body casting and it port may not fit through the hole. A Fein oscillating saw or cheap knockoff will be able to cut that inlet port off and allow it to fit through the hole Safety Googles are a must here.
Good luck