Thank you for the kind words!
I don't want to give too many hints on the last room... But what I can say is that everything you'll need to solve are all in the epilogue map and the epilogue map only! You can also use the stone to teleport to two areas, both of these areas are connected to the puzzle, and if you don't know where to start, maybe you can try interacting with the door again, there is a very important clue on the lock itself
Don't be too bothered to solve it though, all it contains are some behind-the-scenes content which I imagined most of players don't really care, so I made it a bit more difficult to reward those who are willing to put time into solving it, no matter what method you use. (So you can also just crack the game file and get the password through an editor if you are really stuck, I don't really mind xP) I'm glad to hear you want to explore all of the game, I really appreciate it!