No, you won't get your acceptance rescinded for failing a final. Your final grade is what goes on the transcript and is subsequently what USAFA sees. And, I'm pretty sure that since you are already accepted, they don't go back and look at final grades after the fact. I think you're fine.
You are incorrect if you think USAFA does not check final grades. We absolutely do, and we will contact the high school guidance counselor of every appointee to make sure that nothing has happened in the final semester that may require a closer look. Every year there are a few appointees that will have their appointments rescinded in late May or even June for one reason or another.