Generally, drain plugs are at the lowest point so if you remove them all the water should drain out. At least enough so freezing will not be a problem. As for the bubbles, the pool people probably used an industrial version of the shop-vac which blows a large volume of low pressure air, compared to a compressor that provides higher pressure at low volume. Pipes burst when you have ice forming with no place to expand. Picture a length of horizontal pipe that droops in the middle. If there is standing water, enough to fill the inside diameter of the pipe even for a short distance, and the ends of the water freeze first, they form plugs and then the middle freezes. The water in the middle expands, has no place to go because of the ice plugs at the ends, and the pipe bursts. If there is just a little water left in the pipe so it can expand vertically then the pipe will be OK.
Hope this helps.
Al