I have seen leaches migrate through wet grass in long lasting damp weather. They march right out of a swamp or pond when conditions are right. You may hope your pond is a closed system, but it isn't.

Not to be critical, but in a swimming pond, I think the chlorine was a bad move. When reacting with organic compounds in the pond, who knows what nasty things are created as a result. Chlorine gas only being a minor one.

I'm also not so sure you will ever get rid of the copper unless it goes insoluble and is buried under sediment. If you treated with enough to kill leaches, it must have been a substantial dose. You are just going to have to wait it out, then put in some fish that eat them as the best way to keep the numbers down.

Good luck, I hope it turns out well in the end.

A possible solution is to start over. Drain, dry, and enlargen the pond, thereby removing the copper and winding up with a bigger place to swim. Expensive perhaps, but could turn it into opportunity.