I've had the core game for a bit now, but with the recent Kickstarter, I got my hands on... well, everything else. There's two things that are giving me trouble:
a) What components from 1.5 are added to the original set, and what parts are meant to replace?
b) The Arena and 6-player set add more cards and tokens. Should those be restricted from normal play? If so, how?
you should replace all cards from the v1.0 of the game that have the same title as the v1.5 cards, and simply merge the rest. For example the "Pushback" action card in the v1.5 should replace the "Pushback" action card from the core game.
The only exception here is the new card for the boss "Black Carrier". We have issued an Errata about this card, you should keep the original one and throw away the new one.
As far as the six player expansion is concerned, you simply merge everything with the original game. No need to restrict them or remove them when you are playing with less than 5 players.