If you were at a city council meeting, would you address the mayor as “Mr. Mayor?”
No word games about it. Just a title of respect.
Not if I didn't have to. Again you are trying to slip in a sleight of hand. A man made political office does not a religious doctrine make. But to answer your question, "Render unto Caesar that which is Caesars", so I would. But I would NOT call this pope "His Holiness". That is forbidden. God does not want me to bestow any artificial praise on him to suggest he is any holier than the next guy (and he isn't).