Kanaplex with the copper too?
From @Humblefish explanation you stop the powerhead before the fish goes in. So it's just to aerate the water a head of time.
Ok to take him out of copper, into FW dip and back into copper?
Yes, kanaplex is OK with copper. Yes, for a 5-minute fw dip it's fine to pull the powerhead after aerating. If you were doing the chemical bath which is 60-90 minutes I would continue aeration during.
And yes to the last one as well. If he doesn't survive the dip there is a good chance he wouldn't have made it regardless.