The fact the returns are plugged is a GOOD sign. Can also look if the pump has the drain plug pulled or is dry, and same with the filter. As for the liner, finding small tears can be a PITA. Best way is to fill it and see if the level drops. At its present level you can try and give a good visual around the waterline because if it was leaking the leak would stop at the present level. Stairs are a pretty common place for leaks at the pinch strips or whatever they're called.
On the leak test jig, if I don't get the pic thing solved I can always email stuff to you when the time comes.
Al