Hey! thanks for playing. Sorry that you're lost. The keys can still be found even if you have all the monsters and their trophies.
Some hints:
Some of the monster's need to be dead in order to get their clue.
Try interacting with some of the monsters' backgrounds with the care items (dryer, soap, brush).
On the computer in the back room you can find a list of recently edited capsules/monsters. This lets you know which monsters are involved in key puzzles.
For some reason in that list I forgot to include the mermaid, so I'll be fixing that too.
If you want anymore hints feel free to reply to this comment. Again thank you so much for playing! It means a lot.
Also thanks to your comment I was able to find a bug!