I'm splitting up my response to this into several pieces because it got long.

The key bit, IMHO:

So to answer A, I'd say 'there is no fundamental property of 'containment'. It's just a word we use to describe one thing surrounded by another in circumstances X and Y.

And I would agree with you.

and that leaves the question 'can it contain things': [..] The same is not true of consciousness, because it's not (just) a function.

"No," replies A, "you miss the point completely. I don't ask whether a container can contain things; clearly it can, I observe it doing so. I ask how it contains things. What is the explanation for its demonstrated ability to contain things? Containership is not just a function," A insists, "though I understand you want to treat it as one. No, containership is a fundamental essence. You can't simply ignore the hard question of "is X a container?" in favor of thinking about simpler, merely functional questions like "can X contain Y?". And, while we're at it," A coninues, "what makes you think that an artificial container, such as we build all the time, is actually containing anything rather than merely emulating containership? Sure, perhaps we can't tell the difference, but that doesn't mean there isn't a difference."

I take it you don't find A's argument convincing, and neither do I, but it's not clear to me what either of us could say to A that A would find at all compelling.