You don’t really need to do anything to repair the wound, in five years it will just be that part of the tree that looks a bit knotted. Nature has very good healing strategies, and any “help” we give is more often than not just getting in the way.
Re: training, I put three varieties on here and I am mainly going to let one grow out, I will bend out two of the 5’ or so shoots that grew out of it over this year on that variety and also one smaller shoot on one of the other varieties to make a 3-branched vase.
In general I am training all of my dwarf apples now to what I call “umbrella” system, there are few smaller shoots in the first 5’ of height and all the fruit is in the 5’-7’ area. It makes it more work for the deer to get the fruit if its all high up.