The key in Right hand's room is from the chest in right hand's office, which is in the main castle on the top floor. That should give you the 16th letter and you should be able to listen to him right there to get the 11 (because he's asking for key to his room, which you already found in the Tailor's room). Thank you again for playing it:)
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I did it! That was really satisfying! Loved the last little puzzle and the whole idea of the secret ending. I was sort of hoping to find Whiskers somewhere but I was still happy to see a drawing at least! I've not often seen castles in games that have such an authentic-feeling layout like yours. Thanks for helping me, I hope I wasn't too bothersome. Happy holidays!