Goats are pretty hard to overdose on copper. I'm sure it's possible, but they aren't like sheep - any supplemental copper is too much copper for sheep and can kill them...goats, not so much.
My guess is that it was barberpole worms as well.
My [then 6 month old] wether almost died from them, and lungworms, right after bringing him home [goats, especially young goats, should be dewormed immediately after experiencing major stress, like moving, and I didn't know].
It was really only luck that he survived!! I noticed that he was getting "bottlejaw" [possibly the swelling you saw?] and that he was a little lethargic. The next day he was down and couldn't/wouldn't get up. LUCKILY my goat-mentor-friend had let me know that, with those symptoms and how I hadn't dewormed when the goats came home, I needed to deworm asap.
I used Valbazen, plus doses of horse-RedCell every 2 hours, and he came around within 24 hours.
He still coughs after exerting himself, due to what the lungworms did to his lungs, but overall he's fine - 4 years later.
Mostly, once you start seeing symptoms like bottlejaw, the goat is already nearly dead [bottlejaw is caused by SEVERE anemia].
I'm sorry for your loss! Goats, especially young goats, are very tricky at first. Don't blame yourself, goats are not easy. *hugs*