Ok, so do you have any idea when the new tank is coming yet?
What if you turn the new tank into a temporary QT and administer hypo there? Move all of the fish over (maybe minus one pair of clowns, the perc pair could probably go in a 5-gallon instead)
Flukes have a fallow period of 4-weeks, so you would only have to keep them there for that long. Or, if you held them a bit longer could possibly take care of the ich while you are at it... provided the ich does not survive hypo (some strains have been known to be resistant)
Another option would be to treat with hypo first, and then CP after bringing the salinity back to normal range. I would avoid copper though since the hope is to use that tank as a DT.
Would just mean Dory would need to wait quite a while for her permanent home
Otherwise, if you could find a other larger tank second-hand that would work too.