1) discard the scupper, even if it means installing a new mushroom. What size is the existing thru-hull. A single thru-hull of the proper size can feed 3 pumps. A thru-hull of proper size can feed 2. This cuts down on bronze hardware and holes in the boat.

2) I do not suggest above the water draining with reversible pumps. Too much extra plumbing and its inefficient at best.