(1) Since they are ( or it would seem to be ) 2 separate tickets, then yes. If it is one single issued ticket, then you would not be charged
(2) you are definately entitled to use the Manchester Terraces lounge on the outbound. At Paphos on the outbound and Manchester on the inbound , I am not sure what BAs policy is when travelling on a separate ticket
(3) Yes. You can go and use the CX Wing or Pier at HKG
Dave