Don't know how you get that from me mentioning how a SINGLE FIN could acquire the ability to move rather than being a static structure from one mutation. After all, having extra muscles or missing a few is actually rather common.
Your entire line of thought is bunk because the capacity to move predates multicellular organisms. That is, there is no reason that the first multicellular organism wouldn't have had the capacity for movement to begin with. You act as if the capacity to move has to independently evolve over and over for no apparent reason, as if how a single cell moves can't possibly contribute to the genes that allow a multicellular organism to move. Heck, even sea sponges, which don't have a nervous system or muscles, have the capacity to move. It becomes more complex over time, stop acting as if the movement had to start with the fish.