Listening to part 52 for moments where we hear the piano where I don't think Kayne's sitting at the piano:

"I am going to kill him" and then "The Wager" drop. Piano's there. either he sat down right after getting the stone and is playing that entire time John and Arthur are arguing with him, or someone else is playing --

piano plays quietly while he's killing Lillith - physically could not have played that while his fingers were in her head.

Look I think he's standing and gesturing dramatically up until after he kills Lillith and John says "he's walking back to the piano and sitting down"

gets up again at "Is that so little duckling"

piano doesn't play again until the "a citadel of bones" line. features while Arthur's forgiving John and in the lead up to "I have my Wager to think of"

no piano during the start of the dismemberment I think! begins again at "you are a shell of a man" Only instrument playing during the eye gouging until the final note.

so, Kayne wasn't physically playing the piano a good bit of "The Wager."

I tend to think he forced the ourthurchestra to learn new instruments and that they didn't already know those instruments, but the piano is giving me pause. If an ourthurchestra member was playing the piano, that's interesting. Still a removal of identity in a way - having to cede the piano to Kayne at moments, the fact he's now playing again while someone else dies, and parallels slightly to intermezzo and part 23 - but in a different way than the other ourthurchestra being forced to play something else. He still gets to play the piano. The soul of that playing is removed though.

If Kayne's playing the piano via magical means, oh. Because then, it's like, he'd rather no one play this if Arthur can't. The piano belongs to Arthur Lester and to Kayne and these other, different, failed Arthurs don't get that privilege. The piano bench is empty. It still plays, because of course it does. Arthur Lester might have spent ten years avoiding touching the piano, but the piano is very much a part of him. Kayne physically sits down and plays his song after killing his daughter, a reversal of the last night Arthur composed anything. The piano bench is empty. The keys move. John's the one with the eyes, able to see the obvious lack of pianist there, Kayne having sole dominion over an instrument that means so much to Arthur. (Perhaps, in a world where Arthur did say to give the stone to Kayne, the bench wouldn't have been empty.)

okay I'm now leaning towards Kayne playing it the entire time, with magic or physically depending on the moment.

how did the other Arthurs learn their music? Did Kayne give them their eyesight back to learn the music? Did he make them memorize it and then take away their eyesight for dramatic effect

(see? So easy you could play it BLIND *cue manic laughter*)

Or did John have to learn how to read music? I bet that went down well, with their varying arrays of communication skills. OR did Kayne just make them Magically Know the music (boring)