If the filter is a bit clogged, just swish it slightly in a bucket of tank water and put it back in the filter. I'd not clean it too much so there will be enough beneficial bacteria to keep the nitrogen cycle under control while your fish are battling ich.
Hot water and salt will work just fine. Vacuum the substrate thoroughly to remove possible ich cysts that are stuck there. Hard surfaces harbor your beneficial bacteria.
One teaspoon per gallon every 12 hours with a total of 3 teaspoons per gallon. You're on right track so far.