I would try to obliterate the willows no matter what it takes. I have gone down and tied a rope to them and pulled them with a tractor.

On the knocked down brush it depends on what type of water you have as to whether they are suitable. I did that with fresh oak and the tannin came out of them and poisoned my water. I tried stocking and it was lethal. I had to pump the pond dry. Cut a piece of it and put in a bucket of pond water. If it turns the water black, burn them.