People in the comments have just taken it as a given that consciousness resides solely in the brain without explaining why they think this. My point in this post is that I don't see why we have reason to reject the 3 possibilities above. If you reject the idea that consciousness could reside outside of the brain, please explain why.

Simply put: Occam's Razor, and lack of evidence.

More thoroughly: We have at best very weak evidence for an afterlife (non-verifiable near-death experiences, religious texts, and it being a generally common belief/hunch that it may exist), plausible explanations for why each of those weak sources of evidence would exist without a real afterlife (psychological effects of almost dying and general unreliability of minds in extreme situations, the fact that an afterlife makes a religious meme more powerful, and flat out hope/unwillingness to face the end respectively). All descriptions of a true afterlife conflict massively with the testable and prediction giving knowledge that science has found, and trying to make the two work together, if it is even possible, would give a much less simple theory.