Until I read this post I had never even heard of fin-rot, so that shows the extent of my knowledge regarding fish diseases, parasites, and ailments. I researched some photos of it, and I do think your photos look similar to the advanced cases that I saw pics of.
Is it possible that the bite marks you're seeing, if they are bite marks, happened after the fish became afflicted? A sick, probably slow moving fish would present an attractive target for a snapper.
I have watched snappers stalk my fish during feeding times, and it appears to me that they prefer to attack from underneath, especially when the targeted fish is free-swimming, or not stationary in shallower water, such as when they are nesting.
I have no doubt that an opportunistic snapper would utilize any method it could, on any fish that it could, to try and get a meal. However it seems unusual to me that they would concentrate their attack, and therefore inflict damage upon, the fins or tail only. From what I've witnessed, a large snapper knows the best way to grab a fish and hang on to it, until it succumbs.
