Totally agree with this: you end up with two possibilities, and one will hopefully be simpler, less contrived, and fit the story more.
I can go along with this in principle – I haven't seen Braveheart for twenty years or so and it comes down to what backstory is given in the film as to whether a) a benevolent occupation is seen as feasible given the way the Scots and English are portrayed, and b) whether the various border violence and death from before the occupation are shown as either a serious problem or glossed over. Assuming the latter – that there's a sense that things were tolerable until the English occupied Scotland (as opposed to things were fine until English Lords started sleeping with Scottish wives and doing other awful things) then Situation as the OS domain is simpler and cleaner.
I'm just trying to show that the choice between Activities and Situation here is only self-evident absent evidence that perhaps the occupation wasn't a problem until the problematic activities took place, or, conversely, that even prior to the occupation things were intolerable because of conflicts across the border.
It's not a matter of aiming for the demonstratable precision and accuracy of mathematics. I'm in fact arguing the opposite: that it's not entirely as self-evident as presented.
To bring this back to Civil War, just observing that there's a lot of punching and kicking going on doesn't mean that's the source of the conflict. There's no means of removing that conflict, so just saying, "stop the problematic activities" becomes a bit like, "end all crime and everything will be fine" without the movie ever providing a viable means for that to take place (i.e. somewhat like your argument that it's rather difficult to envision the English in Braveheart suddenly becoming benevolent occupiers.)
This is precisely my point: to stop the problematic activities, you'd have to get the aliens, monsters, robots, and super villains to stop too. Nowhere in the movie is that shown as possible. The only way to unstick the situation is to get people to let go of their binary mindset ("you're either with us or against us") and work together to reduce the casualties that come with these conflicts.
Look at all the times in the movie where, if they just stopped for a second and considered the possibility that what they so firmly believe might be incorrect, that they could have avoided the conflict:
A bomb goes off at the U.N. and dubious evidence appears showing a photo of Bucky. The government says, "There he is – let's go get that guy." Captain America says, "Wait, I don't think he'd ever do that." They respond with, "We don't give a shit, we're convinced he's guilty, so let's go get him." Steve knows that with that attitude, they'll never end up taking him alive, but will end up killing Bucky, so he has no choice but to get in the way. If they just stopped and considered the other side's arguments, they'd come up with a means of detaining Bucky without the risk of everything going to hell, but they won't.
When Bucky and Black Panther encounter each other, Bucky says, "I didn't kill your father", Black Panther says, "Then why did you run" and they fight. If they just stopped a second to consider that their attitudes might be wrong – if Panther asked, "Well, what evidence can you give me that you weren't the one who did it?" or if Bucky even said, "Yeah, running did make me look guilty, but I figured no one would listen since I've done tons of terrible stuff in the past", they might have come to some agreement as to what to do. The actual source of the conflict – the belief that Bucky set off the bomb – never gets dealt with in that encounter. This conflict only happens because of one thing: T'Challa wrongly believes that Bucky is the man who killed his father.
The airport scene has the same problem: the refusal of either side to consider that their belief about the primacy of the new rules of the Accords might be wrong prevents them from just stopping for the five minutes it would take to come up with a better solution than fighting.
Take away the fixed attitude, and there's a way forward. Keep it, and you've got half the heroes retiring, meaning less capability of countering all those violent situations, and the other half operating as soldiers. As Steve Rogers asks early on, "what happens if they tell us we're not allowed to get involved when trouble comes – that we're not allowed to save those lives." (I'm paraphrasing.) He's making the point that the Accords won't stop the casualties.
So that's my argument for the OS domain of Fixed Attitude. While it may be wrong for any number of reasons, when I consider the litmus test, it looks like the story problems get resolved. There's never any means of ending all the problematic activities, so I can't see how you can get everyone unstuck through Activity.