Hear! Hear! appears to be a shortened form of "Hear ye! Hear ye!"

Not shortened. The earlier form is "Oyez, oyez!". The more natural translation of that is "Hear, hear!". The English use in Parliament might date from the time English was adopted as the official language... I should know when... late 1300s?

Town cryers are said to cry "Hear ye", but how authentic a tradition is that?