This is a GREAT topic!! If I had time, I could write 10,000 words on the subject, and still not be done. Since I am on a tight schedule, I'll try to be concise.
First off, I too struggle with the whole MC concept. For example, I'm working on a WIP in which there are two POVs. One, I think could reasonably be argued is the MC. But the second POV is prominent, crucial, and in some ways more intriguing than the MC. So what should I call him? I wish there was some standard designation to indicate a VERY important character who still doesn't quite rise to the level of the MC. Maybe, AMC--Almost Main Character. (Just kidding.)
And then there is an issue I struggle with in, thankfully, only a small minority of my mss. Before delineating it, I should mention this list I have of the Ten Most Common Writing Errors, put together by a World Class Editor. I have found it indispensable, in improving my writing. And guess what the single most #1 writing error she encounters is: I'll let her put it in her own words:
1. Whose Story Is This, Anyway? The Plight
of the Protagonist: The biggest single plot problem I see in my judging, editing, and critiquing is actually a character
problem: the passive or undermotivated protagonist-- that is, a protagonist who is not truly involved in causing the plot to unfold. Beware of the victim-protagonist (bad things happen to him, and he
suffers a lot), the passive protagonist (he witnesses the plot events, but he doesn't participate), the bumbling protagonist (he acts, but stupidly, without learning from his mistakes). The central character doesn't have to be likeable (though it helps) or (God forbid) without faults, but he does have to be motivated enough to act and encounter obstacles and change in response to plot events.
Ideally, the protagonist should be involved in nearly every event, and his decisions and actions should drive the plot. You might make a list of all the major plot events, and beside each note the protagonist's contribution. Is each action or decision or choice motivated? (The motivation doesn't have to be laudable, but should derive from who he is and what he wants.) Does each action have some effect on the plot? And finally, does each action-event dynamic contribute to an ultimate change in the protagonist?
Here's a final thought that might help you plot: One primary purpose of the plot is to force the protagonist to change, usually by recognizing and overcoming some internal conflict. Know your character, and you'll figure out your plot. Conversely, know your plot, and you'll find the character who needs that sequence of events for internal growth.
That was a big quote, but here is the bottom line. Every now and then I gravitate toward a character who observes everything, thinks and feels a LOT, but is too passive to fit the description above. It is a VERY hard problem to overcome! [For example, in one of my favorite stories ever, there is an uncle, a brother and a sister. The story is told through the eyes of the sister, but the brother and uncle are more active in making the plot unfold. Toward the end the sister performs the critical, outcome-changing action that brings the whole plot/story together. Is that enough to change her from passive to active? I don't know.]
Anyway, I could say more on the subject but I'm about out of time. I'll just add, Sunstruck, that I too wrote a story with four POVs that involved the characters growing, etc. I hope yours isn't as dark as mine; one of the characters was perniciously suicidal.
Thanks for posting such an interesting question! I hope many writers weigh in.