Ever consider not QT? I have some biologist friends that swear it stresses fish badly. I don't QT and haven't had an issue. I do understand the risks associated with this practice.
Well you yourself answered your question so I’m not sure what your asking.
Yes copper is a poison to fish and it can do irreversible damage if administered incorrectly. However, when dosed properly the dangers are minimized and all but eliminated if the fish maintains its appetite. And while you knowingly risk and gamble every time you add a fish to your tank, others choose not to for various sound reasons. I choose to QT everything I get simply because I am often away for days if not weeks at a time. And as we all know our tanks like to take a turn for the worse when we aren’t home. Any little “stress” trigger could then compromise my fish’s immune system and the suppressed disease would then do more harm and damage than 30 days of copper.