Hilco,
I am going to use a lot of Haskell concepts here, but I am a strong believer that using the correct terms empowers the learner to go out and continue their education on their own.
Your first question is is it possible to define Result as Result success failure and still create an instance of Functor?
The answer to that is yes. We can look up the definition of the Functor typeclass (use :i Functor in GHCI), and we can see the following: class Functor (f :: * -> *) where....
>From that definition, we can see that we can have an instance of Functor for any type that has the kind * -> *. It seems like you are familiar with treating type constructors as functions at the type level, so what we are saying here is that in order for a type constructor to be a functor, it has to be a type constructor that takes one type constant to yield another type constant. Your type Result has the kind * -> * -> *. It takes two type constants, so we know that wont "fit" into Functor, and we have to partially apply the Result type constructor to get another type constructor with the kind * -> *, which can have an instance of Functor.
What I think is missing from your understanding is this - if you change your data declaration to be Result success failure, then your instance of functor will only be able to modify the failure case. Why? Because when you write your instance of functor, it will look like this: instance Functor (Result success) where.... If we look at the signature of fmap :: (a -> b) -> f a -> f b, and then we specialize so that f ~ Result success, we get fmap :: (a -> b) -> Result success a -> Result success b. In other words, you have already baked in the first type argument to Result when you are writing your instance of Functor for it, and are not allowed to change it anymore.
Your second question I would like to be able to express that "result" is not touched...
I am assuming here that you are talking about this piece of code "fmap _ result@(Failure error) = result" using your original functor instance and data Result failure success...
We can't express that result is not touched. Why? Because result is touched. It has to be. As you yourself mentioned, on the left-hand side we have a value of one type, and on the right-hand side we have a value of a different type. Even though we don't witness the change at the value level, we do have a change at the type level. So we can't express it because it is not what is happening. Now, I get what you are saying - the result on the left-hand side has the same data as the right-hand side, same data constructor, etc, but it is still not the same value.
On your third question, regarding GHC being smart enough to realize a NOP, I don't know.
I hope this helps!