@mlucas , I love this thread because it's got me thinking of the MC and Storymind, both in ways I haven't before. I love that. Anyway...
I suspect the MC test is a lot like the Litmus Test for determining Domains and Throughlines in that it doesn't present you with a definitive answer but it does offer you a "maybe".
I think the issue with the MC test is that by comparing what and when the characters know something with what and when the audience knows something, it ends up inadvertently tying the audience mind to Storymind. But the Storymind is separate from the audience mind. I think it's even been discovered to be separate from the authors mind (couldn't find the thread where this happened, i think most of you know the one I mean though). The Storymind is a metaphorical human mind, but is individual from either the author or audience and unique unto itself just as I am from you.
To see what I'm trying to say, let's look at the CA:CW example (really glad you used that one. It's the exact scene I was thinking of all afternoon but wasn't sure I wanted to bring up again). The MC test is a good way to see if Tony might be the MC, but Tony isn't the MC because we find out about his parents at the same time he does thus allowing him to pass the MC test. He's the MC because it is through his character that the Storymind personally experiences what it's like when I find out that Bucky killed MY parents. It's just the nature of this personal experience that we the audience get that information at the same time.
Steve, then, isn't the IC because he already knows about Bucky's involvement in the death of Tony's parents and we don't. He's the IC because it is at him that the Storymind looks (from the viewpoint of Tony) to ask "What did you know?" And "Why didn't you tell me?" Again, it's just the nature of the YOU perspective that means we the audience didn't know that yet.
Just for fun, let's flip the MC and IC. If Steve were the MC, that scene would have been about the Storymind's experience of what it's like when I'M present as my good friend Tony learns that hisparents were killed by my best friend Bucky. And if Tony were the IC, it would be at him the Storymind looks (from the viewpoint of Steve) to ask "How are you going to handle this information? What are you going to do about it?" I'm sure an argument could be made that this is what's happening, but I don't think that's what that scene was about.
So back to the original topic, I think the audience is reading/watching the Storymind's experience of the I perspective. So if your MC is an unreliable narrator, I think that changes the flavor of the experience. Instead of the experience of a reliable narrator saying "this is what I did to cover my tracks after committing a crime, and here's all the steps I took", the experience of an unreliable narrator might be "this was the lie I told or the secret I kept to myself about the crime I committed" or whatever. So if the MC is keeping a secret, I think we the audience may not find out until the end, but I think it should be clear by the end that the Storymind was aware of the secret all along.
But I'm pretty much guessing at all that.