If you finish once and want to get the other gender avis, do you get to just restart or do you have to buy the pass again?
There'll be an option to restart the story once you complete the first playthrough (no additional cost required). Make sure to open the avatar boxes before you restart as there's a 50% chance of dropping either fem/masc avi.
The key may not work for you if you haven’t finished some of the tutorial quests when you wrote your post. You need to unlock the events (hunt) feature of the game to have the key work. When in tutorial mode is incomplete “it will just be there “
...so what happens if you get to the ending of the story pass and forget to open the chest before starting the second round of it? I worked so hard to get the finale male stage and forgot to open the chest...now I have the female 4th avi instead of the male. I want to die right now because that's not what I wanted.
I think I'll just emo quit at this point because I was never interested in the female avis. I just wanted some of the items rewarded early on because I know I won't be able to reach the fourth stage a second time since I'm busy irl.