I would NOT return the fish to the main tank, unless as a last resort.
You can generally bring up the temp back to normal quite quickly (say a few hrs ... some would do it even faster). <That is as opposed to bring the temp down - fishies need far more time to acclimatize to cold then to heat.>
Depending on what filters/media you have in the tank, try transfering some bacteria to the QT. Eg. If you have 2 filters, put one in the QT. At the very least, seed your QT's filter with gunk from the main's filter. You should have at most a mini cycle if you seed enough.
I keep my QT's filter in my main tank all the time. That way, any time I need the QT, I have a fully cycled tank.
BTW - regarding ich .... how soon the spots show up depends on temp. At 80's, ich should go through one life cycle in 4-5 days.