Did not get a mount from an event.

[Bebekurenai]

There was no indication that you had to talk to an NPC, and the achievement itself lists the key as a reward for getting the achievement, so why would the achievement list the key if I was supposed to talk to someone else to receive it? Seems like backwards logic.

It's an achievement reward, not an event reward. You should still be able to get the key even today. You are answering yourself here. Did you not get an achievement reward before? If not, then better sit back hehe because the NPC will have a few in stock for you
As for no indication about that NPC, wrong, you get introduced to him at some point when you start the game, so yes, you should have met him and therefore known that as an achievement reward he would be the person to see >_>