Still can't find the key for the tiny door. I don't think Captain Alpaca drops the last key.
I just tested it again and didnt have any issues. From Captain Alpaca you should have received the Newfolk Basement Key
You use the key on this gate at the entrance of Newfolk castle and follow this path to an elevator.
At the other end of the elevator you will find the Soma Key for the optional boss. :)