Short answers, pulled from the story.
An ambassador is the highest-ranking diplomatic representative a state sends to another sovereign state or international organization. Under the Vienna Convention on Diplomatic Relations of 1961, ambassadors hold the top diplomatic rank and carry plenipotentiary powers, meaning full authority to represent their government.
An Ambassador Extraordinary and Plenipotentiary is the full formal title carried by most ambassadors on foreign postings as head of mission. The term "plenipotentiary" means the diplomat has full authority to represent their government, while "extraordinary" historically distinguished ambassadors sent for a special purpose from those on permanent assignment.
The Vienna Convention on Diplomatic Relations was signed in 1961 and took effect in 1964. It remains in force today and establishes ambassadors as the highest diplomatic rank.
Ambassadors represent their head of state in countries outside the Commonwealth of Nations, while High Commissioners represent the government in relations between Commonwealth member states. The distinction exists because Commonwealth nations share or have shared a common head of state, so the standard ambassador-to-head-of-state accreditation does not apply.
Declaring a diplomat persona non grata means the host government has labeled that person an unacceptable presence in their country. This declaration typically results in the ambassador being recalled to their home nation.
The modern ambassador system developed from the Italian Renaissance, with its use as a deliberate political strategy taking shape in Italy during the 15th century. Smaller Italian states used the system to protect themselves through information sharing and alliance-building, and the practice spread to Europe during the Italian Wars.