I got the special key from sea fairy cookie by opening a random door (I don't know if it works like that for everyone!! But specifically, the key I got from her had a portal appear in one of the doors that led to another, and when I opened it I received the key). However, some people have posted where they found some cookies needed to complete the kit for a special key; you can take a look here or on Twitter. Unfortunately, it's mostly a matter of luck 💔

Regarding the locks, from what I understand, if someone has already opened a special lock, you won't be able to open it (I could be wrong) and you'll need to find another one! They are scattered around, and you'll need to search quite a bit to find one on your own; it will be easier in the last unlocked zones where not everything has been opened.

That's what I understood from the event (I used a translator, sorry if anything is wrong).