You do not need to buy Episode 1 if you purchased the Season Pass. The unlock key that is currently associated with Episode 1 is linked to an expired PS+ membership. Uninstall Episode 1 and then reinstall the game content and the unlock key (which will be associated with your Season Pass purchase) and you should be able to continue your game. Reinstalling Episode 1 should not affect your save game.
If you have any question regarding the Instant Game Collection or how ownership of these games works through PS+, please contact Sony Support.