Papal Nuncio
no
The full title of an ambassador is "Ambassador Extraordinary and Plenipotentiary."
Yes, he was given the title in response to his denouncing of Martin Luther.
The proper title for a retired ambassador is "Ambassador (Ret.)"
A Nuncio is an ambassador for the Vatican.
I assume you are referring to Charlemagne who was given the title of Holy Roman Emperor.
The pope gave Charlemagne the title of Holy Roman Emperor, on Christmas Day of 799(?)/800
He was given the title of Holy Roman Emperor.
Nuncio
An ambassador is addressed as His Excellency or Her Excellency, unless they have a royal title. In the case of a royal title, it can be the ambassador who determines their preferred form of address.
This is a title of the Pope which emphasizes his humble role.
The Bishop of Rome became known as the pope or Holy Father.