Don't be so quick to assume it is ich just because it's covered with white dots. May be some skin reaction to the drop and little patches of dry mucus. YOu may see it like it's spreading and growing while it may only be that some skin is pealing off later on.

I would give the fish a week or so and see what happens. It is nearly impossible that after a copper treatment and 12 weeks you would still have some ich. Did you desinfect nets and containers? How can it possibly have survived?

this may be a stupid suggestion, but have you tried UV running on your main tank/qt tank to kill free swimming ich? Its worked for me in the past, well worth the investment, considering livestock loss etc.