Is there a listing of all of the expansions, with the dates of their release? For example, I've seen games described as a "fifth saga expansion", a "fifth deluxe expansion" so is there just "a fifth expansion"?
Must/should the expansions be purchased and played in order?
The community wiki should cover most of those questions
The only order restrictions are Thematic (if you are wanting to stick strictly to the story), and card dependencies when an Adventure Pack requires it's corresponding Deluxe Expansion to work. Otherwise you can play any adventure in any order you want. Though smaller card pools can make some adventures quite difficult.
Never heared about a fifth saga expansion...
You haven't picked up The Flame of the West yet?