That's... hmm. See, Dramatica isn't terribly picky about the mood or tone of your story at any point except the ending. Speaking in four acts, you can have Success-Success-Success-Failure or Failure-Failure-Failure-Failure, and Dramatica sees it as a Failure either way. What Dramatica is good at is constructing a meaningful, interwoven argument within your story, and that's through its four throughlines.
What you're calling the A-Story sounds a lot like Dramatica's Overall Throughline. B is the Relationship Throughline, and C is the Main Character Throughline. So there's just one Throughline you're missing for the complete package: the Influence Character Throughline. (Let me know if you don't know what the Influence Character is.) Here's what I mean by interwoven:
- The Main Character is tied into the Overall Throughline via their Unique Ability and the Critical Element. The MC has some special trait or power that gives them access to the big switch that determines Success or Failure.
- The Influence Character is tied into the Main Character Throughline via their Unique Ability. The Influence Character's chief goal is to make the MC change--whether that leads to success or failure, the IC may not actually know or care. But their whole thing is influence, so influencing is what they're gonna do.
- The Main Character and Influence Character are tied together via the Relationship Throughline. The dynamics between the MC and the IC within their relationship is what makes the subjective part of the story work. In a romantic story, the love between them is (typically) what gives the story its heart and meaning, but in other stories, it can be a father-son dynamic, or a cautionary figure's warding away.
One more thing I need to cover, and then I can answer your question more clearly. Dramatica separates the Outcome, an objective look at whether the characters Succeed or Fail, and the Judgement, which is more of a subjective look at whether the MC has purged their demons or collected new ones (like Pokémon!). This gives you the ability to classify stories into four types: Good Successes, your run-of-the-mill smiles-and-rainbows ending; Bad Successes, stories where the characters technically reach their goal, but at a heavy cost; Good Failures, where the characters fail to reach their goal but are happier for it; and Bad Failures, which are your doom-and-gloom. sucks-to-be-you endings.
So to answer your question, we can have a Main Plot that ends positively, but an MC arc that crashes and burns. Nothing at all wrong with that--I'd give Doctor Horrible's Singalong Blog as an example, if you've seen that. You can have the central relationship end, but still reach success, and the MC is happier for it. (Perhaps Ibsen's Doll House, where the MC's decision to end her marriage is the best thing that could happen to her at that moment?)
But again, Dramatica has nothing to say about Defeat-Midpoints and False-Victories or whatever. You could take a random storyform and tell it any number of ways--of a character who wins and wins, only to trip at the finishing tape and lose it all in one fell swoop, or of a character who makes one mistake at the beginning of the story and pays for it for the rest of the story, or of a character who wobbles back and forth between success and failure before finally teetering failure-ways and succumbing to darkness. Dramatica has no guidance on that front, and that's for the better, I say!