@ 1 tablespoon per 10 gallons for the duration of treatment most plants are OK. This is a bit high for some salt sensitive fish, too, but they can handle it if you add it this way:
Day 1: 1 teaspoon per 10 gallons. Dissolve the salt in water and pour it in slowly, over several hours.
Day 2: Repeat.
Day 3: Repeat.
@ 2 Tablespoons per 10 gallons many plants will start to show problems. This is a higher level than needed, too.
Do water changes that emphasize vacuuming the substrate and other surfaces. This removes a lot of the ich that has fallen off the fish to reproduce. You do not have to remove large volumes of water, but do this every day or every other day.
Add salt to the new water at the rate that is currently in the tank.
It's hard to do much vacuuming of a soil substrate, unfortunately. I did get one good vacuum that mostly removed the chladophora layer, but I won't get much more. I'm starting the heat treatment, but I'm worried because the tank was initially at 68 degrees, so it's going to be some time passing through the real danger zone while the fish acclimate to higher temperatures. Hopefully salt will add extra stress to the parasites in the meantime.
Also, is a sponge filter with a powerhead any use in removing free-swimming parasites? It's just in the sump right now, but I could move it up to the main tank.
Good to increase the temperature slowly so the fish can adapt. Cool water species probably cannot handle it high enough to kill Ich, though (86*F). You can highly likely get it into the mid 70s, maybe a bit higher. This will increase the metabolism of both Ich and fish. Fish can fight it off better, and Ich will grow faster, fall off the fish faster and become susceptible to treatment faster.
Increase the aeration in the tank when you are raising the temperature. Warmer water holds less oxygen. Fish gills may be compromised by Ich in the gills.
I do not think the sponge filter will remove them. I think it will give the adult Ich more places to land on to breed.
Add ultra violet sterilizer to kill the free swimming Ich.
I understand about not giving the substrate a deep vacuum. That is not necessary. The intent is to remove the Ich that land on the substrate and most other surfaces in the tank. They do not burrow into the substrate, though they could fall in between the pieces of a gravel substrate. In a planted substrate (dirt, plant media, other fine materials) the Ich just sit on the top.
Your plants will handle the salt better than the heat. Personally, I have tried the salt/heat treatment only once for ich and I just can't see ever doing that again. Quick Cure has always gotten rid of ich in 3-4 days for me.