I'm probably missing a mechanic, but I can't figure out how to use this key piece in two places on the key?
I need it to liberate the piece from the bottom room, but if I use it then it's stuck in the wrong position for the exit door..
You're not crazy- this is a bug which I passed over twice before patching. The pin in the final door is supposed to match the key fragment with a hole in it. Sorry about that!