It appears you and @Thomas have the full grasp here, but others are following the discussion with interest. For me the possibility that John may not be the same person as 'the disciple whom Jesus loved' is food for thought. Of course the escape valve of 'scribal interventions' is controversial, going back to the Dead Sea Scrolls Isaiah being almost identical to the modern document, as general evidence of Biblical authenticity of transmission? If scribes were editing the Bible at whim as it went along, then any passage can be downgraded as scribal intervention? Not meant in relation to the authorship of John.