Q: I have something tricky in my structure. My impact character is also the hidden antagonist. He is on the protagonist's team through most of the story helping him with the goal. But once the protagonist has it in his hands, the impact character/ antagonist steals the goal out of the protagonist's hands.
Even though I have a STEADFAST/FAILURE/ BAD story for the main character/ protagonist, the CHANGE of my impact character is a positive one since his situation improves greatly since he walks away with the goal.
My question: Is it structurally sound to have a positive change for the impact character even if the main character has a negative (judgment: bad) ending from being steadfast?
A: It's quite alright for your antagonist to appear to be one thing, only to reveal that he (or she) is the antagonist. The way to accomplish this is to make him a complex character with attributes of the antagonist AND some other objective character, such as the sidekick or guardian. This allows for him to function openly in one manner, which keeps him in the protagonist's group, but function as an antagonist behind the scenes. He's always the antagonist, but our awareness of those functions are withheld from us and (some of) the other characters. In this way, he does not "change" his functions as an objective character--which is essential in the Overall Story throughline.
As a Change Impact Character, however, I think you'll find the IC is making the argument to the MC that they're going about things all wrong. They need to lighten, or take a different approach, or whatever it is that the IC domain dictates. However, the MC's steadfastness forces the IC to change, and by doing so the IC adopts the world view of the MC and matches, or exceeds, the MC's ruthlessness. Lucky for the IC, everything works in his favor and he's a happy camper.
The answer to your question is a definite "Yes -- It's OK to have a steadfast miserable MC combined with a Change, happy IC." It's not your typical story, but that might make it what you like about it.