Hi, the advantage of two returns is to make the best use of the higher HP pumps. The second return allows for more water flow.

As for the water level, I would double check with the installer on that one. I would want the water about 6" below the area of the new return hole.

A hole will have to be cut through the wall, with a hole saw I assume. I am not sure how that can safely be done with the water holding the liner up against the wall. If the second hole is already there I could see doing the rest with the pool full.

Enjoy the new pool, Dennis