To be honest, I did once see a very mild case of ick in one of my tanks, only a few spots. I did the heat treatment without salt and it did work.
Fish are said to be much more tolerant of salt than ick, salt can also help the fish with possible infections from the ick. I do believe the salt to be beneficial with removing ick.
I am sure the plants are not loving it, but a couple weeks of it should not kill them and it sounds like the clowns are making a quick improvement.
I would just stick with it.