Hmmm, that's an interesting idea, have to put some thought into it to wrap my head around how well it would work. I don't know the answer to your question about the main drain, but I don't think the ice in the pool would be an issue for the fish as long as you can make some holes in the ice to allow air in. Poconos does this in his pool, I think he just partially weights some 5 gallon buckets to float in the water and lets the ice form around it, then removes the buckets. Maybe you could do something similar there?

What kind of fish are you planning to put into your pond? I have an 1100 gallon koi pond right next to my pool, and they do fine in the winter, as long as they can get to some source of air. Would you still keep your pump/filter intact to provide filtration?

janet