Most of the reading I have done says that they have debunked the myth that once a tank has had copper you can't use it for a reef so it should be fine as a sump later on.

The 75 that the tang was in is now infected and if you have other fish in there they will contract ich as well. While you may not see the signs of ich on those fish it is most likely that they are strong enough that you simply do not see the symptoms and not that they are immune or ich is not present. Removeal of ich from an living tank is a PITA as it needs to sit fallow for quite some time (8+ weeks as I recall). But if you dont take the time and do that when/if you put the tang back they are just going to pick it up again.

I also don't recall inverts being a carrier as the parisite needs the fish to survive, I guess there is a possibility for the water to have some ich if you move the inverts from tank to another.