Hypo salinity is 1.008 and even that may not entirely kill off the Ich. Regardless, the life cycle of Ich is about two weeks, more or less depending on temperature and other variables. I would keep the fish in QT for at least 4 weeks, preferably 6 weeks, after the last spots have disappeared. Make sure your refractometer is properly calibrated if you are going to attempt treatment with hpyo salinity. To high and the treatment may not be effective, too low and it can be damaging or fatal to the fish.
Z, a freshwater dip will not kill spores that are in the tomont stage of development, and may be less than fully successful against spores that are in the trophant stage. It is only during the tomite stage that hpyo salinity and other treatments are truly effective and spores do not transition from stage to stage all at once. A one week treatment followed by introduction of the fish to the display tank is a sure way to transfer the problem to the display.
FW dips are not necessarily a bad thing to do but keep in mind that this will stress the fish and may actually weaken it to the point that it cannot fight off the infection. Do some research on Google or look on RC for the Ich thread that is posted there. Educate yourself and then decide how you want to treat the fish.