Are you equating protagonist with main character? I don't think that's strictly true (although probably is in most cases). I would say MC is the character whose perspective the narrative follows. I think the protagonist is the character who is pursuing the goal of the story. The protagonist can do that without any particular character development though; the POV character (what I'd call the MC) can still undergo a full character arc without being the driver of the plot. Maybe the protagonist acts as a role model (either good or bad) and influences the POV character. In this sort of story, I suppose you could say that the external plot centres around the protagonist, but the internal plot centres on the POV character. And all that is completely separate from who the narrator is (if the narrator even is a character)! Trying to think of an example... you could have an epistolary novel comprising a series of magazine articles about a fallen-from-grace celebrity who has to do community service. The plot revolves around a little girl with cancer that the celebrity meets in doing so. The protagonist could be the little girl (the external plot could be 'will she survive or not?'); the POV character/MC could be the celebrity (maybe the little girl's struggles gives perspective on his own life); the narrator would be the journalist writing the articles.
Would you have a problem with your story having both characters be the stars of an ensemble cast? I love it when stories have multiple main characters instead of just one main character and a bunch of side characters. One of my favorite things about Frozen was that Anna was the lead protagonist and Elsa was the secondary protagonist, and I always found it annoying when I'd read people complaining in reviews that they couldn't tell who "The Protagonist" was between the attention paid to the both of them. Even Buffy the Vampire Slayer tended to be just the first main character among equals in her stories, and it was magnificent.
I kind of see where you're going, but to use that analogy I would say the external plot centres on the secondary character, while the internal plot centres on the 1st person narrator. In other words, character B has his problems, but it is character A (the 1st person narrator) whose struggle we're following - albeit a struggle brought on by character A's problems.
In the case of your story, yes, exactly because in your story the POV character is the narrator (while your 'secondary character' might be the protagonist). I was speaking more generally in my previous post, acknowledging that the POV character doesn't always have to be the narrator (e.g. 3rd person narrative).
