[jmillikin]

  > Therefore no communication system can exist that meets
  > your criteria. (E.g. Because the CPU could be compromised)

For the purposes of this discussion it's reasonable to assume that consumer hardware does not contain backdoors, because such extensive compromise of the computing infrastructure would require conspiracy on a massive scale (approximately every electronics manufacturer in the world).

[embolism]

Then you haven't explained how Apple could join another device to the encryption session without the user's password.