There's a lot of stuff to unpack there and terms that are getting conflated. I'll start by confessing that I'm not a scientist, but having at least basic exposure to science in university, I'll do my best.
In my life I've never heard of "evidence via consensus". You have evidence and then it gets interpreted and that can lead to a consensus. Evidence is, pretty much by definition, outside of the interpretation.
Psychology validates hypotheses through empirical studies. It's not done simply by theorizing or debating, but by devising experiments specifically designed to produce an objective measure (the evidence) that can then be interpreted.
Things may have changed since I was taking psych courses in university, but back then, there were no studies that showed that psychiatry was more effective at managing mental illness than any other active attempt on the part of the patient to seek help (e.g. seeing pastors, talking to spirit guides, or whatever else) except for drug therapies.
And doing "hard science" specifically means empirical studies, not philosophical reasoning or debate. When psychologists are performing 'hard science' on a 'soft subject', they're subjecting theories to rigorous empirical experiments.
Again, a couple of things are really getting conflated here. You're interchangeably using the Dramatica conceptual term of the "story mind" with a human mind. You talk to, argue with, and have a conversation with a human because that one person is the only one who can describe their thoughts. That's what's meant by 'what's on someone's mind.'
Dramatica isn't a person. You can't "talk to, argue with, or have a conversation" with a movie. You're having it with other people, none of whom are inside the movie or the figurative 'mind' that you're referring to.
I don't think this metaphor is serving you well here, which is why it's generally not a good idea to construct a metaphor from something that's similar to what it's attempting to refer to (describing a cat through the metaphor of a cartoon cat isn't likely to work well.)
Look, there's a movie. It's a real thing. It exists, can be watched and can be measured. It's delivering a story – a subjective experience interpreted by the audience. Some aspects of a movie present a widely shared experience: most people watching Die Hard can tell you when you're seeing the hero versus when you're seeing the villain. Other aspects of a movie are less widely shared, such as whether the cause of the breakdown in the hero's magic is his fault for his actions or his wife's for not accepting him for who he is (or vice versa). Then you have a storyform – an abstract construct intended to map out the underlying characteristics of the story. Some aspects of a storyform will be easily discerned by those examining the movie, for example, almost everyone would readily identify the MC of Die Hard as Bruce Willis's character. Other aspects will be less widely accepted, such as whether there's only one IC (the cop), or two (the wife). I'm not saying it's the latter – I'm just saying it's not as self-evident.
The question is whether there's a single, absolute, objective storyform that can be discerned from a finished movie.
No, because the psychiatrist doesn't pretend to know with absolute certainty what your thoughts are even after talking to you. And the psychiatrist who goes and debates it with ten other psychiatrist also doesn't know with absolute certainty what your thoughts are. They work on probabilities. Given what the patient is describing and other measurable factors, there's strong evidence to suggest diagnosis A. There's also evidence that it could be diagnosis B. Or C. They choose one based on a balance of probabilities and the relative risk of applicable treatments.
There's a term for a psychiatrist who claims that through conversation they can discern with absolute certainty the precise thought in your head: a quack.
I'm not saying that for Dramatica to have value any five people have to instantly be able to independently achieve the same storyform after watching a movie. I'm saying that without that you need to be very hesitant about insisting that there's a single, absolute and objective storyform behind a movie.
Dramatica provides tons of insights and benefits in the story creation process. It also poses fascinating and illuminating questions about finished works – ways for someone to reconsider what they thought they saw. You don't have to treat it as a religious certainty to derive value from it. But if you do want to insist that it provide the one true explanation for the underpinnings of a film and that this manifests as a single, correct storyform, you aught to be able to demonstrate that with at least some attempt at empirical objectivity.