No medication or treatment can kill ich when it is showing up as encrusted white spots on your fish. The only stage in which ich can be killed is in their free swimming stage when they're looking for a host. If you are following the directions exactly, just keep treating according to the instructions.
Also, if you introduce the salt slowly enough and acclimate them to it, it shouldn't be that stressful on them. I treated it that way a while ago when I had pristella tetras and 5 months later, they're perfectly healthy.
Some other info that I'm not sure applies to medicated versions of ridding ich is that you have to keep up treatment for 7 days AFTER seeing the last white crust fall because during their life cycle, they will be born (in the crust), fall to the ground still in the crust and protected, and finally be born and break the crust to swim in the water and find a host. This whole process happens in days, though the speed at which this happens will be increased if the temp is higher. Seven days is a good enough number to catch them all in their free swimming stage and leave a few days extra just in case. It is important to do it right the first time and make sure you kill them all because one can become 200 in only a few days. If you do it right the first time, they will never come back, unless the tank is reinfected from a tank that is already infected.