To add a little to what Janet said about floating the liner. As soon as the water level in the pool goes below the ground water level the thing can float. So, if you do need to drain some to dilute water or for whatever reason try and do it when the surrounding ground is dryest.
On the main drain and light, they are another source of leaks and problems. A main drain makes keeping the water cleaner easier and lights are nice. If I ever get the ambition I'd tap into the skimmer line and add valves and a suction feed just off the pool deck so I can hook a homebrew thing to drop in the deep end and perform the function of a main drain. Take it out when the pool is occupied.
Al