I don't know if you can get Tricide Neo, or its twin Neocide 3, but those are good water borne triple antibiotics that are good for treatment when injectibles are out of reach. The use of the broad spectrum treatment can be done with the salt level that you have, though it is off label. John Fornaro of Hanover Koi Farm about half way down this page https://www.hanoverkoifarms.com/koi-treatments/ discusses his testing and use of Proform C (same as broad spectrum treatment) with salt at very high dosages. So to keep from having to remove all of the salt before treatment. I don't know what has happened to the fin, but it looks similar to some net pulled fins that I have seen and it will settle back down over time, but keep a very close eye on it. The peeling could be slime coat build up and shedding, and this could be caused by costia or some of the other parasites, which will be killed by the broad spectrum treatment.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard