Everything is impossible until it isn't, but as far as I know, there haven't been any doctors or patients willing to attempt a human brain transplant (i.e. full-body transplant) for many reasons: connecting nerves (as you mentioned), connecting blood vessels, possible immune system rejection (even in genetically identical cases), not to mention all the ethical baggage.
Power Vacuum includes some Michael Chrichton-esque science fiction, in that it plays with concepts that sound plausible but may be decades away from realization. So yeah, I'm like 90% sure young Hunter is old Hunter. I was the first one to point out the surgical scars after all. But for now I consider it just a theory, 1) in case I'm wrong, and 2) in case I'm right (to preserve some sense of surprise).