Abdullah I was first Emir of Transjordania (1921-46), and then King of Jordania (1946-51). Similarly, his brother Faisal I was King of Iraq.
My friend, who studies Arabic, said "emir" denotes a political/military leader who doesn't need necessarily to be independent. I wonder, therefore, if the Hashemites adopted "Malik" to represent their independence, and if any Arab leader even used the term before them.
To strictly answer your second question, Hussein bin Ali of Hejaz ruled as a malik in 1916. I think it's an old old Semitic term implying any kind of lord, but has become much more prestigious in the 20th century.