Ambassador Airways ended in 1994.
Ambassador College ended in 1997.
If the Ambassador is currently active in the Diplomatic Service, it would be Mr. Ambassador or Madame [ Mme.] Ambassador.
Dear Mrs Ambassador..
On the Envelope His/Her Excellency Mr./Ms./Dr./Rev. [name] Ambassador (or High Commissioner) of [country in full] In letter Dear Mr/Mrs Ambassador
An internuncio is a papal envoy or messenger but ranks below a nuncio, or ambassador.
Ambassador.
attashe
Best I can do on this one: Since the person is not officially an ambassador yet, you would address the letter as follows: Dr. John Smith Ambassador Designate to Oklahoma <<at his present address>> In the greeting of the letter, refer to him by his present title: Dear Dr. Smith, etc. In the opening of the letter, it is always proper to congradulate him on his appointment. After his becomes official, then you would address the letter to: Honorable Ambassador John Smith Embassy of the United States 123 Any Street Omro, Oklahoma 12345 and in that greeting: Dear Honorable Ambassador Smith...
The Delome letter
The de Lome letter.
How about "agent"