Without wanting to reignite this whole thing . . .
My sense is still that there are two throughlines, but I've laid those out before so if they aren't warranted then they just aren't.
Assuming one storyform, then to me the simple fact that we see the story unfold primarily through Cap's eyes to me makes him the main character. Again, though, I think there are enough important moments that we're being forced to see things through Tony's eyes that it keeps feeling like a second storyform in the same way as Jerry McGuire (to me, anyway.)
Given the whole thing about initial driver having primacy over defining the storyform, I understand why the OS has to be in Activities and why under that principle Zemo is the protagonist.
My sense now is that Cap is the MC and in Situation ("A man out of time"). His personal problems don't stem from the things he believes (which are how he connects to the OS), but rather of the fact that he was pulled from his own time. He loses Peggy Carter - the one person he still had when he found himself in this time, and then is at risk of losing Bucky. There's pretty much nothing for him in this time period except his best friend and the Avengers, and both are at risk of being taken away. In terms of his attitudes, they all worked perfectly fine in his own time. The emphasis made in the movie is that this isn't World War II anymore. Steve's living in the future now, and the future is too complicated for him.
Tony's problems don't stem from his situation of his parents being dead – they come from his attitude about the kid who got killed because of him in Sokovia. Furthermore, he's a total be-er to me. He tries as hard as possible not to have to take action in the movie. He just wants to adapt to the new regime set out in the Accords and do what he's told. He only takes action because Cap's do-er actions (running off to help Bucky, chasing after Zemo...etc.) force him to.
To Jim's questions:
Because him protecting Bucky has nothing to do with trying to stop everyone from ending the Avengers. The initial driver as you identified it is the Scarlet Witch incident. The story solution is to break up the Avengers. Capturing Bucky really doesn't move that one way or another. They could have shot him dead in act 1 and you'd still have the conflict over the Accords.
Part of protecting Bucky is not admitting that Bucky killed Tony's parents. This is a lousy situation for Cap to be in: his oldest friend is responsible for the deaths of his best friend in this time.
As I said above, I don't think he's in Fixed Attitude. He's been pulled out of his own time and is stuck in the future. Send him back to the 1940's and he'd be fine.
I just want to emphasize again that I think the decisions the filmmakers made to force the audience to see both sides (in effect, to make it sometimes feel like "I" is Steve and other times that "I" is Tony), creates cognitive dissonance between the objective storyform of the script and the subjective experience of watching the movie. If they'd made Whiplash with the intent that we see things from the teacher's perspective half the time, we might still have the same storyform but would, I think, be confused as hell.