Sure, no problem. I'll preface here by saying I'm going to include in my reasoning things that hopefully align with Dramatica principles but also things that are possibly completely irrelevant from a Dramatica point of view. This will allow for separating out the non-Dramatica considerations (or re-stating them more accurately within the Dramatica model.)
1. In this movie, MC and IC are sometimes muddled.
I'm starting here because I think it helps explain in part why there's so much debate about this. Despite being called "Captain America: Civil War", you could just as easily (and possibly more accurately) call it "Avengers: Civil War". The Avengers movies, like the comics from which they're derived, tend to be that most dastardly of things, "ensemble" stories.
In some scenes in the movie, the main character appears to be Steve Rogers: when he's at the table seeing the footage that makes Scarlet Witch start to cry, "Enough," he says, both troubled by what he's seen and trying to look out for his people. We're not watching Cap through Tony's eyes, we're right there along with him. The "I" is Steve Rogers. What is Tony doing here? Pressuring him to change his approach: "I can live with whatever restrictions they want to put on us. Why can't you?"
Other times, the main character appears to be Tony: when he's confronted by the woman who's son died because of the Avengers. We're seeing the story through his POV in that moment, wrestling with what Tony's wrestling with.
So, all I'm saying here is that it sometimes appears as if the MC throughline ends up being a handoff between Steve and Tony as we varyingly switch to their POV's. It creates confusion – not just in terms of analyzing the story, but sometimes just when watching the movie. I think this is actually intentional on the part of the Russo brothers: they want to make it difficult for us to empathize too strongly with the point of view of either character, so they obfuscate who it is by constantly switching the sense of who's experiencing the problems first-hand and who's pressuring the other to change the approach (i.e. "I" and "you" seem to get swapped around)
2. If I've got to choose, then Steve Rogers is the MC and Tony is the IC
Here are reasons, some dumb, some hopefully not:
We spend most of the movie with Cap, not with Tony. This is a function of pure math of onscreen time (i.e. "turn off the sound and what do you see?")
Jim mentioned the idea that the audience knows what the MC knows. While I have issues with this (based on Dark Knight, Serenity, and The Contender), it seems to me that Tony keeps a lot more secrets from the audience than Steve does. Tony's manipulated all these things in the background such as having Vision subtly hold Scarlet Witch prisoner (we don't find this out until Vision is forced to admit it – Tony hides it from us), Tony puts together a plan to stop Steve from escaping with Bucky which we don't know about until he suddenly ambushes them at the airport – unlike Steve who's told us his plan to steal the Avengers quinjet to get them out of the country.
The movie's called "Captain America: Civil War" and in every one of those movies, the main character has been Captain America. It ain't called "Tony Stark: Guilty Conscience". Of course, a filmmaker can set out to make one movie but end up making another, but having unintentionally made Tony the main character would have to be seen a colossal failure to achieve their intent. This isn't Sherlock Holmes – we're not watching Steve through Tony's eyes.
(by the way, I know this is a weak point and contradicted by the whole ensemble thing, but I wanted to include it for the sake of completeness)
- Cap isn't trying to force Tony to change his position. He constantly says to him, "I understand why you feel that way, why you've made that decision. I just can't make it with you." It's Tony who consistently pushes back, unwilling to accept anything less than Steve changing his approach. Isn't this the actions of an influence character?
3. MC Domain of Activity, IC Domain of Manipulation
Let's start with the internal/external: Steve is a do-er. I really don't think there's any possible way to view him other than as someone who tries to solve the problems by enacting his will on the external world. He doesn't try to adapt to the Accords by "getting over himself" or changing how he thinks. He continues to do what's right instead of what others try to force him to do.
My sense is that the MC is in Activities. I'm tempted to say the concern is in Doing, because Steve's conflicts all come from the things he does: the fighting, running, helping Bucky escape . . .etc. However a strong case could be made that his overriding concern throughout the movie is to prevent Bucky from being captured and killed. We see that in most of Steve's scenes from the moment we know Bucky's in danger right up until the end of the movie.
The influence character through line is in Manipulation. We see Tony trying to manipulate people into choosing his side all the way through the movie (unlike Steve who often warns them that siding with him will get them in trouble.) He tries to manipulate Steve into signing the Accords with false promises (in the pen scene), he uses Spider-Man's star-struckness to get him to join his side, he gets Vision to try and hide the fact that he's actually holding Wanda prisoner.