Regardless of how long you are in the hobby, there are always more questions.
For the aquarium salt, the salt does not dissipate, so you replace salt only for the water you change. So if you do a 50% water change, that's 10 gallons, so you would add 2 tablespoons of aquarium salt.
As to your medication, I would continue the treatment through the full 7 days before changing the water. If that makes your water changes a few days late, it shouldn't be a problem at all.
I hope that helps. Feel free to ask if I can be of any further help.