Do all expansions have to be bought in pairs to maintain playability for 3-4 players?
Not necessarily. Post the core-set, expansions will be divided into scenario/encounter cards, and player cards. You only need one set of the scenario/encounter cards per play group; however, the expansions will only come with 1 playset of each player card.

So, if a new player card comes out and you and your friend both want to run a full playset of it, you'll need two copies of the expansion. But if you can play around that, you only need one.